Abstract: The dynamic response of a simply supported beam subjected to a moving load with variable speeds is quantified by considering both influences of the inertia force of moving mass and its acceleration on lateral vibration. The principle variation of beam deflection is calculated with two variable parameters: the initial speed and the acceleration of moving mass, using the solution method of time-variant mechanical system. Results show that the influence of acceleration on lateral vibration can not be ignored. However, its inertia force has little influence on the lateral vibration, which can be ignored to simplify the calculation under certain conditions.