The invention discloses a system capable of automatically adjusting focal length, relates to the technical field of nondestructive testing, and solves the problem of low detection efficiency of power transmission and transformation equipment by use of X ray digital imaging technology. The system mainly includes an X ray machine; an imaging plate; a terminal connected with the imaging plate and a movable base connected with the terminal, the terminal is used for receiving an X ray image of attenuated X ray generated by the imaging plate, and recognizing the image quality of the X ray image; the mobile base comprises a chassis, wheels and a clamping groove; a controller, a motor and a power interface are arranged inside the chassis; the controller is respectively connected with the motor and the power interface, the controller is used for sending motor driving commands and adjusting the position of the movable base according to the state of the image quality; the power interface is connected with a power supply through a power supply line, the wheels are arranged on the bottom of the chassis, the wheels are in transmission connection with the motor, the clamping groove is arranged on the top of the chassis, and the X ray machine is placed on the arranged on the clamping groove. The system is capable of automatically adjusting the focal length and improves the detection efficiency.