With the fiber-reinforced thermoplastic flexible pipe (RTP) used in underwater pipe-lay as the object of the research, considering the effects of wind, wave and current load and the motion of pipe-lay platforms as well as the axial tension and bending shear of RTP, a safety assessment method on the underwater pipe-lay design of the big RTP installation was given by using the catenary theory and hydrodynamic calculation method. The critical parameters of the RTP including material property, laid angle, pipeline shape, top tension, buoyant weight and laid depth were analyzed, the nonlinear problems of RTP and pipe-lay platform under the dynamic action of environmental loads were studied, and the failure modes of axial tension and bending shear of the big RTP were mastered. The results show that the theoretical guidance and failure analysis method can be used to assess the safety of RTP during the installation procedure under sea effectively and truly. Finally some reference advises of engineering practice for safety assessment were proposed.