The magnitude of the original quality and original angle of hypersonic vehicle can influence flying orbit and other design-parameters greatly. Firstly, according to flying process, the flying orbit was divided into six states analyzed by three models. Then a modified particle swarm optimization (PSO) algorithm was used to test the relationship among original quality, original angle and level distance. Referring to the test result, optimization research on a period of flying orbit of hypersonic vehicle was performed. Finally, the method has been successfully used and validated its rationality and validity by testing and analyzing the example.