In this article, a device for measuring inertia moment by using a threeline pendulum method is improved. On the basis of the original device，a crossbeam is installed to place a newly designed threeline pendulum. The lower disc is replaced by a thin hemispherical shell with 3D printed uniform mass distribution. The improved device can be used to measure the inertia moment of a spherical symmetric rigid body，especially for measuring the inertia moment of various sports balls. The experimental measurement of the inertia moment of a thin semispherical shell and a solid sphere are in good agreement with the theoretical values. The inertia moments of basketball，football and volleyball around the axis are measured and compared with the theoretical values when they are considered as thin spherical shells. By measuring the inertia moment of a uniform spherical shell with a certain thickness through experiments，its inner radius can be calculated and high measurement accuracy is achieved. The improved measuring device can be widely promoted in future experiments.