The fourth-generation 45GHz ECR source(FECR) is under developing in the institute of modern physics(IMP).The one core of the FECR is the sextupole Nb3Sn coil. To test the performance of the individual sextupole coil after the process of winding ,heat treatment and vacuum-impregnated,a Mirror structure based on a Bladder and Key technology during the magnet assembly is devised.In order to simulate the assembly procedure from bladder and key pre-stress at room temperature through cool-down and verity the proposed method with Ansys ,a dummy coil according to the size of the sextupole Nb3Sn coil is got.Test results of the strain in the middle of shell and dummy coil exhibits very good agreement with expected results calculated with the structural analysis program ansys,and the maximum error is less than 10%.The result verifies the rationality of the Mirror structure design and the credibility of simulation results.