The invention discloses an optical resonant cavity used for a free electron laser and capable of switching reflecting mirrors. The optical resonant cavity comprises a marble base, a vacuum cavity, multiple reflecting mirrors and mirror holders, a mirror table clamp groove, a six-dimensional precision adjusting platform, a mirror library of the reflecting mirrors and a moving mechanism thereof anda mirror grasping manipulator and a moving mechanism thereof. The vacuum cavity is connected with the marble base and arranged on the marble base. Multiple reflecting mirrors and the mirror holders, the mirror library, the mirror grasping manipulator and the six-dimensional precision adjusting platform are arranged in the vacuum cavity. The six-dimensional precision adjusting platform is vertically arranged and connected with the baseplate of the vacuum cavity. Fast online switching of multiple reflecting mirrors can be realized, and each reflecting mirror is switched to the working position and then can realize five-dimensional independent precision adjustment. The curvature and the coupling output hole size of the single reflecting mirror are fixed and can only cover a small range of oscillator free electron laser output. The optical resonant cavity can be applied to the technical field of infrared and terahertz science, such as terahertz imaging, energy chemistry research, etc.