This paper quantitatively studies the transient dynamic response of a semi-submersible production platform with the loss of one or several positioning mooring lines. A semi-submersible platform, production risers, and positioning mooring lines are all included in the numerical simulation. Increased motion of the semi-submersible platform, tension variation of the remaining mooring lines/risers and the risk of mooring line or riser clashing are all investigated through fully coupled time-domain analysis. Combined environmental loads are selected from irregular waves and the steady current varying from very rough to extreme sea conditions. Three dimension radiation/diffraction theories and Morison's equation are applied to calculate first-order wave force and second-order mean drift force of floating semi-submersible platform. Nonlinear time-domain finite element methods are employed to analyze the behavior of mooring lines and risers. Results show that the failure of mooring lines seriously reduce the platform's stability performance. The tension of the rest lines is also increased accordingly. Remaining lines which are closer to the failed lines will have larger tension increase to compensate. Line-Line distance provides practical information for the risk of clashing investigation.