The relationship between falling time and falling body mass in the experiment of measuring moment of inertia with falling body method is discussed． A novel approach is proposed to measure moment of inertia where the slow-motion shooting function of Smartphone is employed to capture the rotation of rigid body，then the rigid body angle location can be obtained according to the time line of the video． Using Origin to process data，fitting curve to obtain the angular acceleration of rigid body，finally obtain the moment of inertia of the rigid body rotation．Compared with the traditional measurement method，the measurement method is more simple and more accurate，which is of great significance for improving the teaching of physics experiment．