The invention relates to two open loop steroid saponin compounds, i.e. Sinularoside A and Sinularoside B with antibacterial activity, separated from sinularia gyrosa, belonging to the technical field of medicines. The compounds have the chemical structural formulas as the specification, wherein the R group is FORMULA. The in-vitro antibacterial test on the Sinularoside A and the Sinularoside B shows that the compounds have obvious effect for inhibiting fungus, bacteria and algae for test, thereby the two open loop steroid saponin compounds can be used for preparing antibacterial medicaments. The invention provides a new lead compound for developing new antibacterial medicaments and has great significance for developing ocean medicine biological resources in China.