On the basis of analyzing the real model 23mm ЗY23-2, the paper chooses an alternative physics model, building a mathematical modeling dynamics, set up a system of equations and solve to find the cycle of operation on automatic firing system, change some parameters of cylinder and study on its effect to do automatic firing system; besides surveying the effect of gas-vent hole diameter and clearance between piston and cylinder on automatic firing system of 23mm ЗY23-2. The research method is based on the calculation theory to ensure compliance with the manufacturer's gun design and use documents.