The article presents the study of beam longitudinal motion and the physical design of the longitudinal matching devices at HIRFL. Two linear bunchers are used on the verylow energy SFC axial injection beam line and the new idea of halffrequency bunching mode is introduced to compensate the longitudinal mismatching between the cyclotrons. The two bunchers on the low energy beam line are designed to work at multimode for reason of the large acceleration range at HIRFL.