The invention provides a variable combustion chamber volume mechanism of a low-speed machine, and belongs to the field of low-speed machines for ships. A piston pin of the variable combustion chambervolume mechanism is connected with a piston assembly and a cross head, and a connecting rod pin is connected with the small end of a connecting rod. The rotating centers of adjusting rockers are concentric with the axis of the piston pin, the adjusting rockers are fixed to the circumference of the connecting rod pin, and the rotating centers of the adjusting rockers are not concentric with the axis of the connecting rod pin. A first plunger assembly and a second plunger assembly are mounted on the piston assembly or the cross head or a support fixed relative to the piston assembly. According to the variable combustion chamber volume mechanism, by controlling the first plunger assembly and the second plunger assembly, the combustion chamber volume of the low-speed machine is flexible and variable; the mechanism is arranged on a reciprocating part, and the influences of vibration of the low-speed machine on the combustion chamber volume variable characteristic are lowered. The first plunger assembly and the second plunger assembly jointly drive the adjusting rockers to rotate, different combustion chamber volumes are obtained, and the purposes of low fuel consumption, low emission and high power output of the low-speed machine are achieved.