The quantity change of the copies with mismatch bases in PCR products was analyzed.The general formulas of numbers of copies in different modes after certain cycles (n) were gained and the math relation between the minimum(R_n)of the non-base-mismatch copy proportion of PCR products and the efficiency cycle numbers (N),the copy chain length(H)catalyzed by Taq polymerase and the mismatch frequency(f)was concluded by way of substituting step by step:R_n=(1-Hf/2)^N-1(1-Hf) .The formula has a guidance sense for preparing the gene segments used for the gene expression by PCR technique.