Cascade refrigerating system

The invention relates to a cascade type refrigerating system which comprises a refrigerating loop, the refrigerating loop comprises a high-temperature refrigerating loop, a low-temperature refrigerating loop and a hot gas bypass branch, and the high-temperature refrigerating loop comprises a high-temperature compressor, a condenser, a first throttling valve and an evaporation and condensation heat exchanger which are connected with one another through pipelines and form the refrigerating loop; the low-temperature refrigeration loop comprises a low-temperature compressor, an evaporation and condensation heat exchanger, a second throttling valve, a first two-stage converter, double evaporators and a second two-stage converter which are connected with one another through pipelines and form a refrigeration loop, and the high-temperature refrigeration loop and the low-temperature refrigeration loop share one evaporation and condensation heat exchanger. The defects that according to an existing cascade type refrige