This paper aims to compare the impact responses in float-over installation for a gravity-based platform under two different simplifications of shock absorbers. A coupled heave-roll-pitch impact model based on the Cummins equation has been established to analyze the dynamic behaviors of installation system, in which the time-consuming convolution integral is replaced by a state-space model, resulting in a constant parameter time-domain model. In present dynamic model, the Leg Mating Units (LMUs) and the Deck Support Units (DSUs), known as shock absorbers, are simplified as vertical gap springs (compression only) with appropriate damping properties since the heave motion of the float-over installation system is the main contribution to the impact loads. Linear and nonlinear springs are assumed to evaluate how the properties of LMUs/DSUs will affect the heave impact response at docking stage and undocking stage. The influence of characteristics of LMU and DSU on the dynamic responses at docking stage is also addressed through the Poincare map and phase portraits.