Deep water shallow natural gas hydrate (NGH) is a kind of clean energy and has entered the commercial exploitation stage. However, it produces a lot of seabed sediment in the process of large-scale mining, which not only easily causes undersea natural hazards, but also leads to pipeline equipment blockage and high energy consumption in the mining process. A downhole solid–liquid separator can effectively separate natural gas hydrate from sand and backfill sand in situ, which can effectively solve this problem. In this paper, the safety of a downhole solid–liquid separator desander under torsion conditions is determined by a test method. A numerical simulation method was used to simulate the tension and pressure of the downhole solid–liquid separator, and a modal simulation analysis and erosion analysis of the downhole solid–liquid separator were carried out. The experiments showed that the downhole solid–liquid separator could withstand 30 KN/m of torque, and a numerical simulation analysis showed that it could withstand 30 MPa of pressure and 50 KN of tension. The results show that the maximum stress is 116.56 MPa, and the maximum allowable stress is 235 MPa. The modal analysis showed that the downhole solid–liquid separator produces resonance at a frequency of about 93 Hz, resulting in large deformation, which should be avoided as far as possible. Through the erosion analysis, the life of the downhole solid–liquid separator was determined to be about 2.3 years. Numerical simulation and experimental results show that the designed downhole solid–liquid separator for natural gas hydrate can ensure safety.