Considering units starting and network constraints and the concept of optimization period,a optimization model which is a typical multi-constraint knapsack problem is established to solve the selection optimization problem of units starting in power system restoration period in this paper, and the objective of the model is to maximize the total power generation capability. A relative effectiveness assessment based on a improving data envelopment analysis is adopted to select the initial units to be started, genetic algorithms are employed to solve the knapsack problem to determine the most reasonable units be started at the current time. Finally, IEEE-39 bus system simulation result proves that the proposed model is feasible and effective.