The invention relates to a potential utilization organic Rankine cycle natural gas compression device with free pistons and belongs to the technical field of organic Rankine cycle heat energy utilization. A power piston reciprocates linearly in a working cylinder and drives a compression piston through a connecting rod so as to compress low-pressure natural gas into high-pressure natural gas. A low-temperature liquid working medium is injected into gas working medium completing power cycle, the gas working medium after power applying generates heat and is converted into liquid, and the low-temperature liquid working medium absorbs heat of the gas working medium after power applying and is heated to be unsaturated working medium. The compression ratio of compressed natural gas is changed along with change of area ratio of the power piston to the compression piston. The openness of a proportional valve is controlled by a control unit so that the mixing proportion of the low-temperature working medium to the high-temperature working medium in a working medium mixer can be changed. Afterheat of the working medium after power applying and gasified potential heat of the working medium can be utilized sufficiently, heat efficiency of the organic Rankine cycle is increased, and energy is saved while environmental protection is realized.