1. An optimization design method for a chassis structure of an electronic device based on mechanical, electrical and thermal three-field coupling, comprising the following steps: (1) determining a preliminary design size of the chassis structure according to a work environment of the electronic device and a specific requirement so as to establish a preliminary CAD model of the chassis structure for a mechanical finite element analysis; (2) with respect to the established preliminary CAD model, establishing a finite element model first, setting a constraint and a load, and performing a related mechanical finite element analysis based on a requirement, i.e. using a related mechanical analysis software to obtain various structural mechanics parameters and a chassis body deformation, which are compiled into a data file for mechanical analysis; (3) extracting a mesh model information with the chassis being deformed from the data file for mechanical analysis, and performing a transform of a mesh model among three fields and obtaining a mesh model for electromagnetic and thermal analysis; (4) based on the established chassis CAD model in step (1), setting a specific heat and a thermal conductivity of a material of a heat dissipation component, and preliminary thermal analysis parameters like the power flow of a fan and an ambient temperature; (5) importing the extracted mesh model for thermal analysis in step (3) and the set thermal analysis parameters in step (4) into a related thermal analysis software to perform temperature field thermal analysis, so as to obtain a temperature field distribution of the chassis and compile it into a heat analysis data file; (6) based on the established chassis CAD model in step (1), determining preliminary electromagnetic analysis parameters like a resonant frequency of the chassis and an electrical parameter of the absorbent material; (7) performing a electromagnetic analysis on the extracted electromagnetic analysis mesh in step (3) with a related electromagnetic analysis software, so as to obtain an electromagnetic field distribution inside and outside the chassis and compile it into an electromagnetic analysis data file; (8) with respect to the data files of the three analysises of the mechanical analysis, the electromagnetic analysis and the thermal analysis, designing a sample and performing a simulation analysis on it based on a actual engineering condition, then obtaining a simulation error by testing the result of the simulation analysis with an experiment, and correcting the analysis result of the chassis CAD model based on the simulation error; and (9) based on the corrected analysis result, determining whether the design of the chassis meets the requirement or not, if yes, the optimization design is finished and the structure parameters of the chassis are output, if no, the preliminary CAD model, the electromagnetic analysis parameter and the thermal analysis parameter are modified and steps (1) to (9) are repeated until the requirement is met.