The invention discloses a variable capacity projectile optical test device. The device comprises a connecting mechanism, an optical variable capacity projectile and a combustion cylinder, wherein the optical variable capacity projectile is provided with a variable capacity piston, and the combustion cylinder is provided with a cylinder piston; the variable capacity piston and the cylinder piston are connected through the connecting mechanism, and the connecting mechanism is provided with a unidirectional motion limiting mechanism; the unidirectional motion limiting mechanism is used for restricting the variable capacity piston and the cylinder piston from conducting reverse motions during the process of varying the capacitance by the variable capacity projectile optical test device. According to the variable capacity projectile optical test device, the variable capacity experiment can be achieved, and the actual operation of an engine can be simulated well in comparison with the prior art, so that the airflow motion, the temperature and pressure distribution and the mixed gas distribution in a combustion chamber of the engine under the reciprocating motions of the piston are reliably approximated.