A gas liquefaction system comprises a compressor unit, a precooling heat exchanger, a multistage intermediate turbine precooling unit, a first heat exchanger, a final-stage turbine expander, a secondheat exchanger, a throttle valve and a cryogenic liquid storage tank. The second heat exchanger is arranged between the throttle valve and the cryogenic liquid storage tank. The gas liquefaction system comprises a to-be-liquefied gas circuit, a refrigeration working medium circulating process and a precooling working medium circulating loop. According to the gas liquefaction system, under the samesystem power consumption, the final-stage turbine expander expands under the high temperature by changing the liquefaction process form of the cryogenic end so as to obtain the greater cooling capacity; compared with a traditional helium refrigeration hydrogen liquefaction system, a stage of second heat exchanger is further additionally arranged after throttling of the throttle valve so that thedryness of the liquefied gas can be further decreased to zero, full liquid phase running of an outlet pipeline of the second heat exchanger is ensured, the safe effect is achieved, and the equipment cost is saved; and the lowest temperature of the whole gas liquefaction system is improved, improving of the system efficiency is facilitated, and energy sources are saved.