A combination encoding is proposed for handling the time decision variable in a dynamic transmission network planning (DTNP). The time decision variable is implied in the combination encoding, which converts the DTNP problem into a static programming problem. The encoding satisfies 1 to 1 mapping between the phenotype and genotype, and meets the consistency of phenotypic space distance and genotypic space distance, which ensure the search efficiency of genetic algorithm. Moreover, according to the characteristics of DTNP problem, some improvement measures are given for crossover operators, mutation mode, fitness function, penalty coefficient so as to enhance the performance of the genetic algorithm to solve DTNP problem. The proposed method is validated using a 19-node system, and results show that the optimal solution can be obtained within reasonable evolutionary generation.