Provided are a device and a measuring method for measuring an unbalanced moment on a bottom surface of a circular valve core. The device includes a diverging shaped tube, a water tank, a transparent tube, spring dynamometers, laser sources, a circular valve core, and a high-speed camera with a camera stand. Inner shackles and the laser sources are evenly distributed on an outer side of the circular valve core of the device, the spring dynamometers are connected with the inner shackles and with the outer shackles evenly distributed on an inner wall of the transparent tube. The method records an unbalanced state of the circular valve core under an impact of water flow from different orientations with the high-speed camera on the camera stand, the location of the laser point on the outer wall and a tension force of the spring dynamometer are read to calculate a torque of the circular valve core.