Development of a simple fast-solving method based on the popular Morrison approach for prediction of semi-submersibles' motion response is the main purpose of the current research. The oblique seas' hydrodynamic specifications are modeled using small amplitude wave theory. The velocity forces and moments have been shown to be small and are therefore neglected. The damping coefficients are obtained from experimental results while added-mass is evaluated using known experimental and theoretical data. Also the interference effects of the various volume elements of the semi-submersible on the flow field are neglected. Comparison of this approximate method shows quite a good agreement with those carried out by other researcher or softwares except for pitch motion which gives a much higher prediction. Spectral analysis is discussed in detail and is applied in predicting the short-term and long-term extreme motion response. The short-term extreme response is based on 50 and 100 years return design sea states while long-term prediction is based on the anticipated life of the semi-submersible. At the end for verification of the model and for showing the method capabilities, "AKER H-4-2 P" semisubmersible is analyzed. The approximate method results are in relatively good agreement with the output results of MOSES Ultramarine software as shown.