Objective: To observe effects of the drug-containing serum of Bu Shen Zhuang Gu Capsule (BSZGC Capsule for Tonifying the Kidney to Strengthen the Bones) on proliferation of the rat's osteoclasts and tartrate-resistant acid phosphatase (TRACP) activity in vitro so as to delve into the mechanisms of its preventive and therapeutic actions on osteoporosis. Methods: Forty female Sprague-Dawley rats aged three months were randomly divided into high dosage BSZGC group, medium dosage BSZGC group, low dosage BSZGC group, and the control group. BSZGC was orally administered into the rats of high, medium, and low dosage groups at different dosages for 12 days, and isometric normal saline was orally administered to the rats of the control group. The drug-containing serum and control serum were prepared. Osteoclasts isolated mechanically from the femur and tibia of Sprague-Dawley rats aged one week were cultured with medium added with different drug-containing sera and control serum. The growth of osteoclasts was observed under an inverted phase-contrast microscope, and optic density (OD) value of osteoclasts was determined by MTT colorimetric assay. TRACP activity was measured by the diazol method. Results: OD value of osteoclasts in the high dosage drug-containing serum group, medium dosage drug-containing serum group, and low dosage drug-containing serum group was significantly lower than that in the control serum group (P<0.05) with a dose-effect correlation. TRACP activity in high dosage drug-containing serum group, medium dosage drug-containing serum group, low dosage drug-containing serum group was significantly lower than that of the control serum group (P<0.0 1), and no significant differences in TRACP activity were not found among the different dosages drug-containing serum groups. Conclusions: BSZGC can inhibit the proliferation of the osteoclasts and reduce TRACP activity, which may be one of the mechanisms of its preventive and therapeutic actions on osteoporosis.