Disclosed is a reciprocating piston engine, comprising a combined structure with an optimized double-crankshaft and variable compression ratio pistons, characterized in that the variable compression ratio piston is a piston serving as a double-acting hydraulic cylinder, a control valve bush of a slide-valve type directional control valve is fixed in a central mounting hole of the inner piston, and a control valve core is mounted in a rotatory sliding or nut-ball screw manner in a central mounting hole in the inner surface of the piston top; and the double-crankshaft engine is formed by two reverse rotating crankshafts which are coupled by gears to be in synchronous reverse rotation motion together, each piston being connected to a connecting rod shaft of two crankshafts, and a piston control valve driving mechanism being mounted between the two crankshafts.