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This paper focus on the cooling-down performance of the LHRS for a high-low temperature test chamber, and proposed a new method to optimize the cooling-down process of the LHRS based on a quasi-steady-state simulation model to investigated the optimal MR concentration and the shifting strategy of suction pressure during the cooling-down process. Targeted on the maximum cooling capacity, the optimal suction pressure-evaporating temperature curve at a specified mixed refrigerant concentration can be obtained, which can be matched by the sectional suction pressure adjustment in the practical cooling-down process to reduce the overall cooling-down time. As an example, the fastest overall cooling-down time is achieved using the R50/R1150/R290/R600a mixed refrigerant (0.3/0.4/0.05/0.25 b y mole) and corresponding shifting parameters (including two shifting temperatures and three shifting pressures) for the sectional suction pressure adjustment, when the air temperature in the test chamber is dropped from 20 °C to −80 °C. The investigation results can provide good references for the industrial design of a high-low temperature test chamber. The optimization method used in this paper can also provide references for researches on cooling-down processes of other refrigeration systems using multi-component mixed refrigerants.