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 need and, based on the preliminary design size, establishing a preliminary CAD model of the chassis structure for a mechanical finite element analysis; (2) establishing a finite element model with respect to the preliminary CAD model first, and obtaining various structural mechanics parameters and a chassis body deformation which are compiled into a mechanical analysis data file, by setting a constraint and a load, and performing a related mechanical finite element analysis based on a requirement using a related mechanical analysis software; (3) extracting a deformed chassis mesh model from the mechanical analysis data file, and performing a transform of the deformed chassis mesh model among said three fields of mechanical, electrical, and thermal, and obtaining an extracted electromagnetic analysis mesh model and an extracted thermal analysis mesh model, wherein the extracting the deformed chassis mesh model and the performing the transform of the mesh model among said three fields includes the following steps: (3a) combining a chassis mesh and one or more entity meshes having an overlap with the chassis mesh to produce the deformed chassis mesh model; (3b) transforming a middle node in the deformed chassis mesh model into a mesh node; (3c) extracting a node of a board shell element mesh from the deformed chassis mesh model; (3d) extracting a node of an entity element surface mesh from the deformed chassis mesh model; (3e) recombining the extracted mesh nodes into a new mesh element; and (3f) subdividing said new mesh element based on an electromagnetic analysis accuracy requirement and a thermal analysis accuracy requirement; (4) setting a specific heat and a thermal conductivity of a material of a heat dissipation component, and preliminary thermal analysis parameters including a power flow of a fan and an ambient temperature, based on the preliminary CAD model in step (1); (5) importing the extracted thermal analysis mesh model 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) determining preliminary electromagnetic analysis parameters including at least a resonant frequency of the chassis and an electrical parameter of an absorbent material, based on the preliminary CAD model in step (1); (7) performing an electromagnetic analysis on the extracted electromagnetic analysis mesh model 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) designing a simulation sample using the mechanical analysis data file, the electromagnetic analysis data file and the thermal analysis data file, and performing a simulation analysis on said simulation sample based on an actual engineering condition, then obtaining a simulation error by testing a result of the simulation analysis with an experiment, and correcting the simulation analysis result of the preliminary CAD model based on the simulation error; and (9) determining whether or not the design of the chassis meets the specific need of step (1) based on the corrected simulation analysis result, wherein: (9a) if the design of the chassis meets the specific need of step (1), then the optimization design is finished and the parameters of the chassis are output; or (9b) if the design of the chassis does not meet the specific need of step (1), then the preliminary CAD model, the electromagnetic analysis parameters and the thermal analysis parameters are modified and steps (1) to (9) are repeated until the specific need of step (1) is met.