Comparison was made of a tract of virgin saline-alkali soil land (CK) with farmlands different in cultivation history, that is, < 5, 10 ~ 15, 20 ~ 25, 30 ~ 35 and 50 ~ 60 years of wheat or corn mono-cropping, using the space- substituting-time method to explore effects of land cultivation on horizontal and vertical distributions of soil carbon and nitrogen content in the 0 ~ 100 cm topsoil layer of the saline-alkali land in the Yellow River Delta. Results show that land cultivation significantly increased the contents of soil organic carbon, inorganic carbon, total carbon, total nitrogen, and available nitrogen, especially in the topsoil layer, and the effects became more significant with the cultivation going on, but leveled off after 30 years of cultivation. Groundwater table is a factor affecting nutrient accumulation in deep soil layers, to a certain extent. Organic carbon was significantly and positively related to total N, but in farmlands, C/N exhibited a declining trend, which leveled off after 30 years of cultivation. However, the C/N in the farmlands of the region is lower than the average of the country, which indicates that in long-cultivated farmlands, it is still essential to further increase the level of soil organic carbon.