In this study, weightlessness was simulated by -6° head down tilt position. The test of 30d head-down tilt (HDT) bed rest was used to investigate the effects of compound probiotics on gut Bifidobacteria. Fourteen health males were subjects, who were divided into the control group and probiotics group. The control group had placebo while the probiotics group had the compound probiotics on the fifth day. The results showed that Bifidobacteria amounts significantly (p<0.05) decreased during the initial period while the compound probiotics could significantly (p<0.05) increased the Bifidobacteria amounts compared with the control group. Compound probiotics could increase the Bifidobacteria amounts to inhibit the microflora dysbiosis induced by simulated weightlessness.