A nonlinear numerical analysis model for unburied subsea pipe-in-pipe system on a flat seabed is established. The finite element method is used to analyze the effects of initial horizontal out-of-straightness and parameters for pipe soil interaction on the lateral buckling of the subsea pipe-in-pipe system. The results indicate that: the larger the initial defect amplitude and wavelength of the pipeline, the greater the lateral displacement, and the more concentrated the buckling deformation of the pipe-in-pipe system; when the wavelength and amplitude of imperfection is constant, if the soil friction factor is increased, the length of the buckling segment of the pipe-in-pipe system will be shortened. The research is of a guiding significance for the design and engineering development of the subsea pipe-in-pipe system.