Objective To observe the effect of serum of SD rats lavaged by modified Zuojin decoction on the apoptosis and proliferation of human gastric cancer cells and its mechanism. Methods SD rats were gavaged with modified Zuojin decoction to prepare their sera. Human SGC-7901 and MKN-45 cells were cultured and treated with the sera (0, 25, 50, 100, 200, 400) mL/L. MTT assay was used to observe the effect of drug-containing serum on the proliferation of human gastric cancer cells. Immunofluorescence method was used to detect the expression of ki67 after treatment with the drug-containing serum. The effect of drug-containing serum on the apoptosis of gastric cancer cells was detected by flow cytometry. Western blot analysis was used to detect the protein levels of apoptosis-associated cleaved caspase-3, cleaved caspase-8, cleaved caspase-9, BAX and Bcl2 in SGC-7901 and MKN-45 cells. Results The drug-containing serum significantly inhibited the proliferation and induced the apoptosis of SGC-7901 and MKN-45 cells, and the positive rate of ki67 expression was significantly reduced. The levels of cleaved caspase-3, cleaved caspase-9 and BAX proteins in SGC-7901 and MKN-45 cells increased and the levels of Bcl2 protein decreased. Conclusion The drug-containing serum can significantly inhibit the proliferation and induce the apoptosis of human gastric cancer SGC-7901 and MKN-45 cells, and the mechanism may be related to the activation of mitochondrial pathways.