This paper presents a time-domain simulation method using the wake oscillator model for prediction of Vortex-Induced Motion (VIM) of a four columns semi-submersible. VIM effect should be assessed in an appropriate manner since the VIM causes fatigue damage of the floating structure's mooring lines. There is no way, however, to assess VIM phenomenon on semi-submersible type floating structures in time-domain. Then this paper represents the method to simulate the VIM motion to be able to use in the timedomain simulation of a moored floater in ocean engineering topics. This method for a four columns floater is proposed by modifying the VIM simulation method on a cylindrical column type floater. Some empirical parameters in the method are obtained from the systematic model tests used many type semi-submersible floaters. This simulation method is easy to use and to understand since the method is based on a rational physical modeling. The validity of this method is shown comparing the simulated results of in-line and transverse VIM amplitudes with experimental ones.