To discuss the flying gap effect of the metal flyer on the initiating behavior for TATB-based explosives, initiation experiments for PBX-6 and PBXL-7 were performed. The target velocity and shape of the flyer to explosives were obtained using a 1 550 nm photon Doppler velocimetry. The running distance to detonation (RDTD) of explosive samples was gained by a Terahertz-wave Doppler interferometric velocimetry at the center point. The relationship between the experiment data captured above was analyzed. It reveals that the running distance to detonation of the TATB-based explosive changes non-monotonously with the increase of gap. With the gap increasing from zero to 20 mm, there are five stages. The initial stage is named S0, the flyer velocity declining stage is named S1, the free running stage of spallation is named S2, the remerging stage when the main flyer catches up and remerging with its spallation layer is named S3, and the stage when the main flyer and spallation are united as one is named S4. The RDTD for the TATB-based explosive is the smallest when the flyer velocity comes to stage S4, the RDTD at stage S0 is the next, and the RDTD at the velocity declining stage S1 and remerging stage S3 are the worst together. These experiment results suggest that the initiating performance of TATB-based explosives impacted by the flyer is not always better than the gap layer results. The initiation mechanism of explosives by flyer under different gaps is probably related to the target velocity together with the structure of the flyer. The simplex target velocity rising of flyer can’t always make the running distance to detonation of TATB-based explosives shorter. The initiation mechanism of TATB-based explosives impacted by flyer is more complex than the gap layer, requiring much experiment data and numerical simulation for further discussion.