The invention relates to a balance mechanism of a double-module free piston internal combustion engine generator and belongs to the field of energy and power. The balance mechanism of the double-module free piston internal combustion engine generator mainly comprises a first free piston generator module, a second free piston generator module, two reversing gears and balancing poles. The free piston generator modules are arranged in parallel. The two reversing gears are arranged between two connecting rods. Each reversing gear is meshed with racks on the two connecting rods. The reversing gears are arranged in gaps between a motor stator coil and air cylinders arranged at the two sides respectively. The balancing rods are arranged between the reversing gears and the motor stator coil. Each balancing rod is perpendicular to each connecting rod. Racks on each balancing rod are meshed with the corresponding reversing gear. The connecting rods and the balancing rods are connected with each other movably through the reversing gears. According to the balance mechanism of the double-module free piston internal combustion engine generator, the balancing mechanism of the pinion-and-rack type is assembled in the free piston internal combustion engine generator which is arranged in a mode of a double-module structure, synchronous reversing between two piston assemblies is achieved, reciprocating inertial force and torque which are generated in the process of movement of the two piston assemblies are balanced, and the problem of vibration of a system is well solved.