: Computer simulation on random motion of numerous moving spheres is widely used. This paper presents a collision-event-driven approach that efficiently implements the simulation of numerous moving spheres. For each sphere in a system with n spheres, only the detected latest collision events are saved in the event queue, making the queue only sized O(n). Driven by the collision events in the queue, the sphere system moves forward with a running efficiency of )( nkknc 10 + . Applying the above method to simulate the random thermal motion of numerous gas molecules in the gravity field and without the gravity field, we get an ideal simulation result. Key words: computer simulation; numerous spheres; collision-event-driven