The invention discloses an experiment instrument for a three-line pendulum. The experiment instrument is composed of a support and a three-line pendulum. The three-line pendulum consists of an upper circular plate, a lower circular plate, and three suspension lines that are connected with the upper circular plate and the lower circular plate and have equal lengths. Besides, the experiment instrument also includes force sensors and a force-measuring display device; three force sensors are fixed in three through holes distributed in the upper circular plate uniformly along the same circumference; the lower end of each force sensor is connected with one suspension line; and a signal output terminal of each force sensor is connected with the force-measuring display device electrically by a lead. The experiment instrument having a simple structure can be used conveniently; the stress situation of the three-line pendulum can be displayed visually; and the rotation inertia can be measured accurately.