In this work an extensive verification and validation study is performed to evaluate the accuracy and credibility for CFD simulations of hydrodynamic response of a Floating Offshore Wind Turbine (FOWT) with semi-submersible platform under regular waves. A dynamic mooring model is coupled with the CFD code to accurately simulate the mooring system. The surge, heave and pitch RAOs at wave frequency and double wave frequency, mean surge offset and zero frequency surge QTF of the semi-submersible platform are investigated. The numerical uncertainties of the above metrics are quantified based on three sources - statistical, iterative and discretization uncertainty. Amongst them, the discretization uncertainty is identified as the dominant source of numerical uncertainty. The validation uncertainty is finally estimated combining the numerical, input and experimental uncertainties. Most of the metrics have achieved the validation against the experimental measurement within the validation uncertainty except for the mean surge offset and zero frequency surge QTF. The utilization of the coupled CFD-FEM code is proved to be accurate for simulations of FOWT under regular waves. Guidelines for best-practices of the procedure for verification and validation study of CFD simulations of floating structures under regular waves are also given.