Soil acidification has become a serious challenge to farmers in the past decades as a result of acid rain, fertilizer application, climate change, etc. In the year of 2000, about 21% of the arable land soil was defined as acid soil in China. At the same time, about 50% of the agricultural soil the world over has become acid soil. Researchers have been working in fields related to acid soils from the aspect of its biological perspectives in order to remedy and exploit the soil. In the past years the authors have been focusing on the subject of acid tolerance of rhizobium, which may help its legume host, including alfalfa, one of the most sensitive plant to soil acidity, to survive in acid soil. What the authors found in previous studies has shown that acid-tolerant rhizobia improve rhizospheric environment for growth of their legume hosts in acid soil by secreting alkaline substance. Application of a proper amount of calcium (5 mmol kg^-1) enhances growth and multiplication of acid-tolerant rhizobia, which leads to better growth of the legume host manifested in significant increase in the concentration of total N in the plant (p<0.05) as compared with CK or Treatment Ca0P0, i.e. no P and Ca addition. At the same time, the quorum sensing of the rhizobia in the rhizosphere improves, too. Addition of phosphorous, as demonstrated in volumes of researches, stimulates root extension and helps the plant stand a number of environmental stresses. Nevertheless so far few papers are available on effect of addition of P and Ca together on growth of rhizobia in acid soil and hence on tolerance of the host plant to acid in soil. To explore the effects, a pot experiment on alfalfa and an acid-tolerant stain of rhizobia, Sinorhizobium meliloti 91522 was carried out using the dual-chamber culture method. In the experiment, the pots were packed with acid soil (pH4.45), which was amended with Al³⁺ till Al³ toxicity of the soil reached medium in level and each pot was divided into two chambers, i.e. inner and outer chambers. Rhizobia 91522 were inoculated into the soil in the outer chamber, far from the plant in the inner chamber, to test. Results show that within the 97 days after the inoculation the number of rhizobia was all significantly higher in the pots amended with 5 mmol kg^-1 Ca² than in CK. For example, the maximum number of rhizobia in the soil, 1~2 cm from alfalfa root in the pots treated with Ca² but no P was 615 times that in CK. On such a basis addition of P enhanced the effect of Ca on growth of rhizobia, e.g. the number of rhizobia, increased up to 9.4 times that in CK. Therefore, it could be concluded that interaction existed between Ca and P in the effect. Addition of Ca² and P also significantly raised the concentration of N-acyl-homoserine actones (AHLs) - the indicator of quorum sensing of rhizobia in the soil under acid and aluminum stresses. For this effect, the addition of 5 mmol kg^-1Ca² worked better than that of 10 mmol kg^-1 Ca. The rhizobia in the near root area (1~2 cm from the root) and the far root area (6~8 cm from the root) had similar trends in variation of population during the culturing period, i.e. one week after inoculation, rhizobia spread gradually from the far root zone to the near root zone, peaked in population about 30 days after inoculation, and afterwards the populations of rhizobia in both zones started to decline and level off, However, the population of rhizobia was much bigger in the near root zone than in the far root area all the time except the first two weeks after inoculation, showing that the rhizospheric micro-environment of the host plant is also an important factor affecting the population of rhizobia and quorum sensing. It is tentatively held that addition of 5 mmol kg^-1 Ca² and 30 µmol kg^-1 P is an optimal treatment, which helps better survival, migration and quorum sensing of acid-tolerant rhizobia in acid soil under aluminum stress, and hence growth of alfalfa under the aforementioned stresses. The plant in this treatment was 4.67, 3.10, 3.47 and 14.74 times that in CK in root fresh weight, shoot fresh weight, nitrogen concentration and number of nodules, respectively.