The invention relates to symmetrical rocker type large-torque output engine which is characterized by comprising a first cylinder unit, a second cylinder unit, a first push rod, a second push rod, a fist rocker, a second rocker, an overrunning clutch, a power output shaft, a reverse overrunning clutch, a transmission shaft, a drive gear, a driven gear, an auxiliary rocker, a parallel connecting rod, a connecting rod, a crank, an energy storage fly wheel and a crank shaft, wherein the first cylinder unit and the second cylinder unit comprise cylinders, pistons, fuel supply and ignition systems, inlet systems and exhaust systems; the first cylinder unit and the second cylinder unit are symmetrically arranged at two sides of the power output shaft to respectively transmit piston thrust to the first rocker and the second rocker through the first push rod and the second push rod; and the rocking of the first rocker and the second rocker is utilized to output torque from the power output shaft through the overrunning clutch, thereby transforming the reciprocating motion of the pistons into rotation motion of the power output shaft. The invention solves the problem of dead points existing in the crank connecting motion mechanism, realizes the efficient large-torque output of a piston engine, improves the efficiency of the piston engine, and reduces fuel consumption of the engine.