The interpretation of Synthetic Aperture Radar (SAR) images of dynamic ocean scene is extremely complex, owing to the comprehensive mechanisms of ocean SAR imaging. Along-track Interferometric SAR (AT-InSAR) technique provides us a convenient tool to cope with these problems. But the lack of in-situ data of AT-InSAR is one of the most restricted factors for the further study of this technique. Simulation tools, if appropriately based on sound models, can provide extremely fruitful results to improve the comprehension of the physics and to interpret the SAR images. This paper outlines a spaceborne AT-InSAR raw signal simulator for dynamic ocean scene in time domain. Firstly, we adopt the fractal model to construct the large scale wave of the ocean scene, and then the scene is divided into facets (of approximately half the size of a resolution cell) and each facet is given a dynamic behaviour under certain constraint. Secondly, the RCS corresponding to each frame of the dynamic sea surface is calculated with Kirchhoff Approximation method. At last, the spaceborne AT-InSAR raw signal simulation flow, considering the actual procedure of real satellite moving in the obit, is established. In the dynamic ocean scene raw signal simulation flow, the scene of ocean surface and corresponding RCS was updated at each azimuth slow time. In this procedure, the raw signal can reflect the system error, and it will be quite helpful for the SAR system design. And the simulator is also validated by the accordance of the simulation results and the theoretical results. We adopted the MPI (Message Passing Interface) and parallel computing techniques to implement the fast simulation.