The short-term voltage stability (STVS) of power systems is a critical problem especially with a high-level penetration of renewable energy resources including solar and wind power. In order to improve the STVS, load control, i.e., load curtailment, is an effective solution. This paper proposes a trajectory sensitivity-based optimization method to optimize the load control. Firstly, the mathematical model of the optimal load control problem is formulated as a STVS-constrained optimization model. To quantitatively evaluate the STVS, the transient voltage stability index (TVSI) is used. Secondly, a trajectory sensitivity method is designed to calculate the sensitivity of the load control location and the amount with respect to the TVSI. Thirdly, based on the trajectory sensitivity, the optimal load control can be obtained through a step-by-step calculation manner. The proposed method is validated on the New England 39-bus system. Simulation results show that the method can effectively minimize the required load control while satisfying the STVS requirements.