A horizontal self-balancing supercritical reaction apparatus, comprising a pressure vessel, a high pressure air compression apparatus, and at least one reactor arranged within the pressure vessel. The reactor is internally provided with front and rear pistons, two ends of the reactor are sealed by the reactor front piston and the reactor rear piston, a pressure medium is filled between the reactor front piston and an inner wall of the pressure vessel, the reactor rear piston is connected to a rear piston driving motor by a rear piston push rod, the reactor is provided with a water inlet and a water/air outlet which are controlled by valves, the reactor is internally provided with a heating apparatus, and the high pressure air compression apparatus is connected to the inside of the reactor. The present invention utilises a pressure self-balancing system, which significantly improves the stress characteristics of the reactor.