Abstract: The structure of quasi-parallel collisionless shock is numerically studied using one dimensional hybrid simulation. The results show that after upstream protons interact with quasi-parallel collisionless shock, a part of them are reflected and travel far upstream. This leads to the generation of ion beam instabilities which in turn excite large-amplitude right-hand polarized low-frequency resonant waves. These waves are convected back downstream by solar wind and gradually merge with the shock,in the meantime new waves are generated continuously in the upstream. This process is cyclical. For parallel collisionless shock, there exist the large-amplitude right-hand polarized low-frequency non-resonant waves in the downstream. Low-frequency waves in upstream are gradually enhanced while travelling downstream, finally exceed the old shock and form a new shock.