This study is an investigation of the response mechanisms in Axial-Injection End-Burning Hybrid Rockets. A numerical model was developed based on the conservation of mass in the chamber. Since it was not known whether the fuel shape is affected by oxidizer port velocity, several single port firing tests were conducted to confirm the effect of fuel regression shape and ensure the precision of the model. The calculation results show that the slow first-order lag can be explained by the time in which ports merge with one another, and the fast first-order lag can be explained by the unsteady residence time in the chamber. However, there is no hysteresis characteristics in calculation results. Therefore, the hysteresis characteristics are not caused by unsteady residence time in the chamber and port merging combustion.