In this paper, a numerical model of a swept-source optical coherence tomography (OCT) system based on Fourier-domain mode-locked (FDML) laser was described. In this method, all the optical components of the laser were modeled separately and split-step Fourier method was used to solve the nonlinear Schrdinger equation. The influences of the filter bandwidth on the relative intensity noise and phase noise were studied. Based on these results, the signal in OCT system was simulated to investigate the connection between noise, dispersion mismatch and roll-off of the point spread functions(PSF). As a result, a direct understanding for the relationship between the components of the swept-source and the performance of the OCT system was obtained.