In order to improve the retroreflctance of the retroreflect sheeting, the properties of invalid light in micro corner cube prism were studied theoretically. By using the theory of geometrical optics, the outgoing way of invalid light in single cube was analyzed, and the properties of invalid light in single cube corner were given. Based on the single cube, the characteristics of invalid light of cube array were studied, the result that a part of invalid light could turns to effective light was found. The reflection principle, and variation law with the bottom thickness of invalid light which in corner cube array were analyzed. Testing and verifying by the optical software Tracepro, the retroreflection curve of single invalid area and small array were obtained. The analysis and the importance of corner array thickness on invalid light were proved by the simulation results. With practice, suggestions of design corner array were given. The obtained results provide theoretical foundations for improving retroreflection ratio of the micro prisms and reasonable design of micro prisms bottom thickness.