Target genes were amplified with the TD PCR method,two orthogonal experiments were designed to screen out optimal concentrat ions of various components of the PRC react ion system,while to explore annealing time,extending time and cycling frequency.Results show that soil bacteria 16S rDNA amplification system fit for black soil in North China was in 25 l volume,which was composed of 10×buffer 2.5μl,20 ng soil microbial DNA template,Mg²⁺ 2.5 mmol L^-1,dNTPs 0 25 mmol L^-1,0.3 mol L^-1 primer,and 1 5UTaq enzyme.The TD PCR reaction procedure went like first keeping 95 for 5 min to make soil microbial DNA denatualized,95 for 45 s more,65~56 for 60 s,and then 72 for 90 s,and lowering the tem porature by 1 every two cycles; and starting another 10 cycles of 95 for 45 s,55 for 60 s,and 72 for 90 s,and finally staying at 72 for 10 min for extending.