The invention provides an offshore natural gas liquefying system. The offshore natural gas liquefying system comprises a light hydrocarbon separation device, a mixed refrigerant precooling cycle, a precooling main low temperature heat exchanger, a mixed refrigerant copious cooling cycle, a first copious cooling main low temperature heat exchanger and a second copious cooling low temperature heat exchanger. The light hydrocarbon separation device comprises a demethanization tower. Overhead gas of the demethanization tower sequentially makes heat exchange contact with the precooling main low temperature heat exchanger, the first copious cooling main low temperature heat exchanger and the second copious cooling low temperature heat exchanger after passing through a demethanization tower coldbox and an expansion compressor and then enters an LNG flash tank after passing through a main throttling valve, and LNG and BOG can be separated by the LNG flash tank. The mixed refrigerant precooling cycle makes heat exchange contact with the precooling main low temperature heat exchanger. The mixed refrigerant copious cooling cycle makes heat exchange contact with the first copious cooling mainlow temperature heat exchanger and the second copious cooling low temperature heat exchanger. Energy consumption of the liquefying system can be reduced, system pressure is relived, and the propane leakage risk is reduced.