A temperature differential engine device for transferring thermal energy into mechanical energy at substantially constant pressure, consisting of a low-boiling-point medium steam turbine, a heat absorber, a thermal-insulating type low-temperature countercurrent heat exchanger, a circulating pump, and a refrigerating system, which are interconnected to constitute a closed circulating system filled with a low-boiling-point medium fluid, wherein: a. the low-boiling-point medium steam turbine and the heat absorber constitute a low-density-medium heat-absorbing working system, and the circulating pump and the refrigerating system constitute a high-density-medium refrigerating-circulating system; transverse heat transfer is realized through the thermal-insulating type low-temperature countercurrent heat exchanger, characterised in that: b. the thermal-insulating type low-temperature countercurrent heat exchanger comprises a high-temperature end and a low-temperature end; the heat exchanger walls are longitudinally heat-insulated from each other by means of heat-insulating layers within the heat exchanger; the temperatures of the fluids gradually change from the high-temperature end to the low-temperature end longitudinally; and the fluids exchange heat transversely with each other via the heat exchanger walls; c. the low-boiling-point medium flows from the low-density-medium heat-absorbing working system into the thermal-insulating type low-temperature countercurrent heat exchanger, and flows from the high-temperature end to the low-temperature end during which the medium transfers from the gaseous state to the liquid state; then the medium flows through the refrigerating-circulating system, and recirculates from the low-temperature end to the high-temperature end during which the medium transfers from the liquid state to the gaseous state; there exists a heat exchange in a counterflow manner in the thermal-insulating type low-temperature countercurrent heat exchanger between the medium flows; d. the refrigerating system functions to dissipate the heat which is carried with the low-boiling-point medium; and e. the heat absorber placed before or after the low-boiling-point medium steam turbine so as to compensate for thermal energy consumption of the system which is utilized to do work externally by the low-boiling-point medium steam turbine, so that system's thermal energy maintains balanced. a. the low-boiling-point medium steam turbine and the heat absorber constitute a low-density-medium heat-absorbing working system, and the circulating pump and the refrigerating system constitute a high-density-medium refrigerating-circulating system; transverse heat transfer is realized through the thermal-insulating type low-temperature countercurrent heat exchanger, characterised in that: b. the thermal-insulating type low-temperature countercurrent heat exchanger comprises a high-temperature end and a low-temperature end; the heat exchanger walls are longitudinally heat-insulated from each other by means of heat-insulating layers within the heat exchanger; the temperatures of the fluids gradually change from the high-temperature end to the low-temperature end longitudinally; and the fluids exchange heat transversely with each other via the heat exchanger walls; c. the low-boiling-point medium flows from the low-density-medium heat-absorbing working system into the thermal-insulating type low-temperature countercurrent heat exchanger, and flows from the high-temperature end to the low-temperature end during which the medium transfers from the gaseous state to the liquid state; then the medium flows through the refrigerating-circulating system, and recirculates from the low-temperature end to the high-temperature end during which the medium transfers from the liquid state to the gaseous state; there exists a heat exchange in a counterflow manner in the thermal-insulating type low-temperature countercurrent heat exchanger between the medium flows; d. the refrigerating system functions to dissipate the heat which is carried with the low-boiling-point medium; and e. the heat absorber placed before or after the low-boiling-point medium steam turbine so as to compensate for thermal energy consumption of the system which is utilized to do work externally by the low-boiling-point medium steam turbine, so that system's thermal energy maintains balanced.