The invention discloses a fast replacing mechanism for a robot fixture. The fast replacing mechanism comprises a fixed connection member, a sliding sleeve, a driving mechanism, positioning members and a flange sleeve, wherein the right end of the fixed connection member is fixedly connected with a robot flange, the outer wall of the left end of the fixed connection member is sleeved by the right end of the flange sleeve, a cavity is formed in the middle of the fixed connection member, the sliding sleeve is sleeved in the cavity, the driving mechanism is fixedly connected with the fixed connection member, a plurality of inner positioning holes which are communicated with the cavity are formed on the side wall of the fixed connection member, outer positioning holes which are communicated with the inner positioning holes are formed at the positions on the side wall of the flange sleeve, the positions correspond to positions of the inner positioning holes, the positioning members are sleeved in the inner positioning holes respectively, the sliding sleeve can move rightwards or leftwards along the cavity under the actions of the driving mechanism, the sliding sleeve can abut against the inner ends of the positioning members during sliding, and simultaneously, the other ends of the positioning members are forced to be snapped in the corresponding outer positioning holes. By means of matching of the positioning members, the inner positioning holes, the outer positioning holes and the sliding sleeve, fixed connection of the fixed connection member and the flange sleeve is achieved, so that connection of a robot and the fixture is achieved.