The invention provides a multistage refrigerating device and a control method thereof. The multistage refrigerating device comprises n stages of compressors which are sequentially connected in a cascaded manner, n compression effect detection units, n+1 pressure detection units and a control unit, wherein n is larger than or equal to 2. Each compression effect detection unit corresponds to one stage of compressor and is used for detecting the proportion of effective acting of the compressor in each time of compression process; the n+1 pressure detection units are arranged at the front ends and the rear ends of the n stages of compressors correspondingly and between every two stages of compressors and are used for detecting the gas inlet pressure and the exhaust pressure of each stage of compressor; and the control unit adjusts loads of the n stages of compressors according to detection results of the n compression effect detection units and the n+1 pressure detection units. The above multistage refrigerating device can be used for adjusting the loads of the n stages of compressors so that the load of each stage of compressor can be in a proper range, the loads of the compressors are prevented from being too high, damage to the compressors is avoided, and therefore waste of electric energy due to too low loads of the compressors is avoided.