Abstract: Abstract: In order to discuss the gap size effect of flyer to 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 by 1550nm Photon Doppler velocimetry. The running distance to detonation of explosive samples were gained by Terahertz-wave Doppler interferometric velocimetry. The relationship between the experiment data captured above were analyzed. It reveals that the running distance to detonation of the TATB based explosive change non-monotonously with the increase of the gap size. With the increase of gap size from zero to 20 mm, the target velocity of the flyer stay at the gap layer initiation stage named S0, the velocity declining stage named S1 for the flyer, the free running stage of spallation named S2, the remerging stage of the flyer with its spallation named S3 and united as one flyer stage named S4 respectively. The running distance to detonation for TATB based explosives is the smallest when the flyer velocity stay at the stage S4, the result at the stage S0 which means the gap layer initiating status is the next, the results at the velocity declining stage S1 and remerging stage S3 are the worst together. These experiment results suggest that the initiating performance of flyer to explosives is not always better than gap layer. The initiation mechanism of flyer to explosives with different gap sizes probably is related to the target velocity together with the structure of flyer.