The new Nb₃Sn superconducting magnet system for the fourth-generation 45GHz ECR source(FECR) is under developing in the institute of Modern Physics (IMP). Because the magnet is made from Nb₃Sn which is sensitive to the strain and there are strong Lorentz forces between the solenoids and sextupole coils, the mechanical structure design is a big challenge. In order to validity the reasonability and feasibility of the structure design and its simulation results, a half-length prototype of the FECR magnet was developed.This paper presents the detailed design of the magnet structure. The stress distribution and variation of the prototype during assembly,cooling down and charging have been got with the three-dimensional finite-element model and are presented in the paper. The results will be used as reference of the room-temperature magnet assembly.