Provided is a potential-utilization-type free-piston organic-rankine-cycle natural gas compressor, belonging to the technical field of organic-rankine-cycle heat utilization. In the utility model, a power piston makes linear reciprocating movement in a working cylinder, and a compression piston is driven by a connecting rod to compress low-pressure natural gas, forming high-pressure natural gas. Meanwhile, low-temperature liquid working medium is sprayed into gaseous working medium which has already finished the work cycle, so that the working medium which has already done work gives out heat and becomes liquid. The low-temperature liquid working liquid absorbs heat of the working medium which has already done work, with temperature being increased, therefore becoming unsaturated working medium. In the utility model, by changing the area ratio of the power piston to the compression piston, the compression ratio of the compressed natural gas is changed. Through controlling the open degree of a proportioning valve via a control unit, the mixing ratio of the lower-temperature working medium and the high-temperature working medium in a working medium mixer is changed. The potential-utilization-type free-piston organic-rankine-cycle natural gas compressor of the utility model makes full use of waste heat of the working medium which has already done work and the latent heat of vaporization of the working medium, enhancing the thermal efficiency of the organic rankine cycle and achieving the aim of saving energy and protecting the environment.