The invention discloses a device for measuring the rotation angle of a high-voltage isolation switch transmission shaft based on gear transmission. The device is characterized in that the device includes a driving gear and a driven gear; the driving gear is fixedly arranged on the high-voltage isolation switch transmission shaft, and is in meshed connection with the driven gear; a driven gear transmission shaft is fixedly arranged in the center of the driven gear; a rotary photoelectric encoder is fixedly arranged on the driven gear transmission shaft; and the rotary photoelectric encoder carries out counting and direction identification by outputting orthogonal encoding output pulses, so as to realize accurate measurement of the rotation angle and the motion direction of the high-voltageisolation switch transmission shaft. The invention further presents a measuring method of the device for measuring the rotation angle of a high-voltage isolation switch transmission shaft based on gear transmission. The measuring method is characterized by comprising the following steps: determining the transmission angle measurement resolution delta(theta) of a high-voltage isolation switch; determining the gear transmission ratio i of rotation angle measurement of a high-voltage isolation switch transmission shaft; and determining the number of teeth of a driving gear and the number of teethof a driven gear.