The invention relates to a horizontally opposed type high and low pressure power device and a working method thereof. The horizontally opposed type high and low pressure power device comprises a heat collector, an insulation pipe, a gasification reactor, an atomizer, a high pressure cylinder I, pistons, piston rings, automatic exhaust valves, a pressure valve I, a liquid storage tank, a pressure valve II, connection rods, insulation layers, a machine case, a flywheel, a crankshaft, a low pressure cylinder I, a high pressure cylinder II, a lower pressure cylinder II, air inlet check valves, exhaust control valves and a radiator, wherein the high pressure cylinder I, the lower pressure cylinder II, the low pressure cylinder I and the high pressure cylinder II are arranged on the machine case in a horizontally opposed mode at 180 degrees with the crankshaft as the center. The horizontally opposed type high and low pressure power device has the advantages that a work medium is recycled without pollution, heat energy conversion efficiency ranges from 65% to 98%, the output power is adjusted by adjusting the volume and number of the cylinders of the machine as required, injected liquid and output power can be adjusted in the largest power range, the machine is simple in structure, low in manufacturing cost, high in economic efficiency, energy-saving, environment-friendly, low in noise, gravity and vibration and good in stability.