Provided are a mixed refrigeration system and a quick freezing method thereof. The mixed refrigeration system mainly comprises a freezing chamber (16), a contact type cascade heat exchanger (21), a contact type quick freezing system and a carbon dioxide quick freezing circulating system, wherein a tray is arranged in the freezing chamber (16), a contact type cascade heat exchanger (21) is arranged below the tray, the contact type cascade heat exchanger (21) is formed by overlapping an evaporator of a contact type quick-freezing system and a condenser of a carbon dioxide quick-freezing circulating system and serves as a tray support, and the tray is placed on the support. According to the invention, objects to be frozen in different temperature zones are frozen, so that the cooling rate of the objects to be frozen in the cooling zone and the crystallization temperature zone is changed, and the quality of the objects to be frozen is better preserved. The contact type quick-freezing system and the carbon dioxide spraying type quick-freezing system are overlapped, the condensation pressure of carbon dioxide is reduced, and the requirement for high pressure resistance of condensation equipment is lowered.