Tender-assisted drilling system meets the strict requirements of deck space of the drilling platform, and provides a relatively safe and comfortable working environment for the staff, which has been widely used in drilling operations in recent years. The significant difference between multi-floating body and single offshore platform is that there may be risk of collision between the floating bodies under extreme metocean conditions or in emergency of mooring system failure. In order to prevent the collision during drilling operations, the initial gap between floating bodies should be designed carefully and provide a reasonable scheme to ensure the safety of the drilling system and the feasibility of drilling operations. Therefore, based on the three dimensional potential flow theory, frequency domain and time domain numerical simulation of the motion response of TLP and TAD is carried out according to the marine environment of West Africa with extreme metocean conditions, the effect of different initial gap on the motion performance of floating bodies is explored and the mechanical characteristics of the mooring system are analyzed. Thus, the reasonable initial gap between TLP and TAD is determined by comparing the simulation results. In general, the numerical simulation results of tender-assisted drilling system may provide reference for engineering practice to some extent.