The invention discloses a process unit and a method for removing heavy hydrocarbon during the liquefaction process of natural gas. The process unit comprises a precooler, a hydrocarbon liquid separator, an expansion machine, a heavy hydrocarbon removal tower, a second throttle valve, a hydrocarbon removal ice chest and a dethanizer which are sequentially connected. The process unit has the following active effects: compared with a throttle valve, the obtained natural gas has lower temperature by filling the natural gas to the expansion machine for expansion and temperature reduction by utilizing the pressure of the natural gas; the expansion machine can be connected with a compressor or an electric generator to recycle part energy so as to realize energy conservation; an ethane heat exchanger and the hydrocarbon removal ice chest are utilized to realize full exchange of energy in the process unit, so that an external cold source is not needed, investment is reduced, and energy conservation and consumption reduction are realized; the process unit has excellent heavy hydrocarbon removal effect, removes the heavy hydrocarbon component in natural gas as far as possible, facilitates follow-up liquefaction process, effectively avoids freezing and blocking of heavy hydrocarbon and meets the demand of liquefaction of natural gas.