The invention relates to a simple pendulum device. In the prior art, when a pendulum ball is replaced, the pendulum length needs to be re-determined, and additionally pendulum motion easily gets disordered. The pendulum length of the simple pendulum device is not changed. According to the technical scheme, a pendulous device comprises a pendulum ball body and a pendulum rope. The pendulous device is characterized in that the pendulum ball body is provided with a first hole and a second hole, wherein the first hole and the second hole are coaxial, the axes of the first hole and the second hole pass through the ball center, the hole diameter of the first hole is larger than that of the second hole, the intersection plane of the first hole and the second hole is a plane which passes through the ball center, and the intersection plane is perpendicular to the axis of the first hole. The first hole is filled with a circular tube which is fixedly connected with the first hole, the outer diameter of the circular tube is equal to the inner diameter of the first hole, and the inner diameter of the circular tube is smaller than the hole diameter of the second hole. The two end faces of the circular tube are located on the intersection plane of the first hole and the second hole and the spherical surface of the pendulum ball body respectively. The density of the circular tube is smaller than that of the pendulum ball body. The center of mass of the whole formed by the pendulum ball body and the circular tube is located in the ball center of the pendulum ball body.