Fast and efficient evacuation of pedestrians from an enclosed area is a difficult but crucial issue in modern society. In this paper, the optimization of evacuation from a building is studied. A graph is adopted to describe the building layout with nodes representing areas and edges representing connections. The dynamics of the evacuation process in the graph is formulated by a nonlinear discrete-time model at a macroscopic level. To find the optimal evacuation plan, a consecutive differential dynamic programming is developed. It inherits the differential dynamic programming property that solves the value and optimal policy locally. Additionally, it consecutively executes actions for multiple steps in the trajectory, which is beneficial to reduce computational burden and lower optimization difficulty. Simulations on a four-storey building layout demonstrates our method is efficient and suitable for on-site evacuation plan making.