In a high-power laser system, when the surface pressure of the optical film caused by laser plasma shock wave is greater than the adhesion per unit area of the film layer, the film will produce mechanical damage, and in serious cases, the whole system may not work. Therefore, studying the formation mechanism of optical film surface pressure caused by laser plasma shock wave and calculating the pressure is the key to ensure the normal operation of high power laser system. In this paper, by studying the relaxation process of shock wave on optical film surface pressure, a theoretical calculation model of shock wave on optical film pressure is established, and the variation law of pressure with different parameters is obtained, which reveals the mechanism of forming the optical film surface pressure. The calculation and simulation results show that the maximum pressure is 108 N/m2 during the laser pulse, and the pressure decreases with the increase of laser pulse time after the pulse, and the total action time of laser plasma and shock wave on the film is in the order of microseconds. The pressure increases with the increase of incident laser energy, focal length of focusing lens and incident laser pulse width, which increases with the decrease of the distance between the film surface and the focal plane of the focusing lens. The pressure changes more obviously with the incident laser energy and the distance between the film surface and the focal plane of the focusing lens than with the focal length of the focusing lens and the incident laser pulse width.