According to the requirements of pressure resistance and sealing performance detection of deep-sea instruments and equipment under full sea pressure, a 120MPa deep-sea test chamber for ultra-high pressure environment simulation is designed in this paper. The structural dimensions of the test chamber were designed and calculated, including the cabin wall thickness, the cabin bottom head thickness, the sealing top cover, and the locking nut. The strength of the cylinder structure and the sealing cover of the test chamber were verified and analyzed through finite element analysis, which validates the reliability of the test chamber structure. The 120MPa deep-sea test chamber for ultra-high pressure environment simulation designed in this paper can provide important technical support for testing the pressure resistance and the sealing performance of deep-sea instruments and equipment.