High performance electron cyclotron resonance (ECR) ion source requires superconducting magnets to produce high magnetic fields for confining the plasma. A Nb 3 Sn superconducting magnet is under development at Lawrence Berkeley National Laboratory (LBNL) aiming to produce 2T sextupole field and 3T / 0.5T / 2T mirror fields at the plasma chamber. The magnet is preloaded with the technology of “bladder and key” to secure the enough mechanical contact between the pole and winding. Unlike to NbTi magnet, Nb 3 Sn is a brittle and strain sensitive material, the peak stress during the room temperature assembly and after cool-down must be lower than ~100MPa and ~150MPa. The peak stress criteria must be taken into account for the room temperature assembly. In this presentation, we investigate the mechanical properties at each step at coil winding, bladder and key operation, cool-down and energization for the magnet with 3D mechanical analysis. The results show that the stress concentrates at the coil end, and less clearance for the key insertion. To avoid the damage on the Nb 3 Sn during the room temperature assembly, we optimize the structure to keep the coil stress less than 100MPa without any reduction of the clearance for the key insertion.