The invention relates to a knee joint structure used for a lower limb exoskeleton robot. A shank assembly is connected with a thigh assembly through a four-bar linkage knee joint, and a gas spring is further mounted between the shank assembly and the thigh assembly. One end of the gas spring is connected to the thigh assembly, and the other end of the gas spring is in sliding connection with a gas spring sliding groove mechanism fixed to the shank assembly. By utilizing the gas spring to provide assistance, conversion from standing to sitting and conversion from sitting to standing are realized. When a user normally walks, the slip of the gas spring, caused by the bending of the knee joint, cannot reach the bottom of the gas spring sliding groove mechanism, so that the gas spring is not compressed or stressed, and normal walking is not affected. When the user wants to convert from standing to sitting or convert from sitting to standing, the slip of the gas spring, caused by the bending of the knee joint, can reach the bottom of the gas spring sliding groove mechanism, the gas spring is compressed and stressed, and energy is stored, so that assistance is provided for converting from standing to sitting and converting from sitting to standing.