In this paper，Fraunhofer diffraction of plane transmission grating brought by random incidence laser is studied through Fourier transformation theory.The distribution of comparative light intensity is analyzed，and relation of bright brand’s location and incidence laser’s information is achieved.By data processing，the method of computing laser parameters is achieved.On this base，the design of a prototype machine is brought forward.In the design，the diffraction images are observed by CCD，the data system is processed by high speed signal processor.By the argument of instruction’s parameters，the performance of prototype machine is deduced.