In order to investigate two-step detonation initiation process, the pre-detonators with different L (distance between transition section inlet and pre-detonator outlet) inserted transition section are simulated. The results indicate that the smaller the L, the more obvious the enhanced effect of the transition section is, but the effect caused by gradually outspread passage is weaker. When L turns longer, the trend is opposite. So it is necessary to choose a suitable L for the transition section and the gradually outspread passage to optimize the enhanced effect of detonation wave transition. When L is equal to 35 mm, the numerical simulated results indicate that the high temperature and high pressure area can be formed earlier to reinforce the detonation wave transition. The gradually outspread passage between pre-detonator and area expansion can form a better retardant effect on the return detonation wave, which prevents the excessively augment of the pressure in blender of the main combustor tube, then the two-step initiation of detonation can be successfully achieved.