The invention provides a device which uses waste heat produced when a laser works to reasonably eliminate laser coherence. The invention uses heat produced when a laser works to heat phase change working medium, and realizes decoherence process to laser beams generated by a laser via cyclic motion of working medium in an annular cavity; thus, the invention not only reasonably uses resources, but also designs a cooling device with a decoherence device together so as to ensure compact structure of a laser display device as well as obtain better decoherence effect.