The invention discloses a measuring device of the inner wall size of a large rotary workpiece. The device comprises a pedestal, a mounting frame, a rotary mechanism, a longitudinal moving mechanism, a transverse moving mechanism, a leveling mechanism and a laser displacement sensor, wherein the pedestal is used for supporting other parts of the device; the mounting frame is used for mounting the device on a workpiece to be measured; the rotary mechanism is arranged between the pedestal and the mounting frame and used for driving the laser displacement sensor to rotate on the horizontal plane; the longitudinal moving mechanism is used for adjusting the position of the laser displacement sensor in the vertical direction; the transverse moving mechanism is used for adjusting the distance of the laser displacement sensor relative to a workpiece measuring point in the horizontal direction; the leveling mechanism is used for adjusting the posture of the laser displacement sensor to enable laser beams emitted by the laser displacement sensor to horizontally illuminate the workpiece measuring point; and the laser displacement sensor is used for measuring the distance from the workpiece measuring point to the sensor. The invention also provides a measuring system and a measuring method based on the measuring device. The space coordinates of the measuring point are determined through laser triangulation theory, so that the measuring precision is high; and the measuring position can be automatically adjusted. The device, the system and the method are suitable for field measurement of various large rotary shell parts.