Speed control has been found a feasible mean to reduce the energy consumption of belt conveyors. However, the current research has not taken the determination of the acceleration in transient operation into account sufficiently. With respect to the belt tension rating, demanded safety factor and the ratio between belt tension before and after drive pulley, this paper presents an approach to estimate the acceleration time. Case with specific horizontal belt conveyor is studied. The simulations with finite element model are carried out to analyze the belt dynamic behaviors in transient operation. Further simulations are carried out to get the optimum acceleration time