To measure the moment of inertia of rigid body under damping condition, a new method combining machine vision and torsional pendulum method is proposed. According to the measuring principle of the torsional pendulum method, the mathematical model of the pendulum swing angle θ with respect to time t, the damping ratio ζ and the undamped natural frequency ωn is established. Finally, the calculation method of the moment of inertia under the linear damping condition is deduced. A high-resolution industrial camera and an imaging system were used to photograph the torsion pendulum. The curve of the pendulum swing angle θ with respect to time t was obtained by capturing the image edge, extracting the feature points and calculating the pendulum swing angle. According to the curve, the damping ratio ζ, the period T and the moment of inertia I were calculated. A set of experimental system is set up to measure the moment of inertia of the measured object whose moment of inertia is less than 4.00×10-3 kg·m2. The results show that the absolute value of the measurement error is less than 4.55×10-5 kg·m2. Repeated measurement experiments were carried out for four kinds of tested objects, and the standard deviation was better than 1.2×10-4 kg·m2. This method can be used to measure the rigid body inertia in engineering. The Z-axis rotation inertia of an unmanned aerial vehicle is measured, and the validity of the method described in this paper is further verified.