Abstract: Parallel interstage separation of the two stage to orbit (TSTO) reusable launch vehicle (RLV) is a key factor to be considered in the design of two-stage aircraft. Using the general computational fluid dynamics (CFD) software, NNW-FlowStar developed by the national numerical wind-tunnel (NNW) project, the parallel interstage separation process of TSTO under sideslip angles was studied with the anisotropic unstructured grid and overlapping grid technology. The software simulated details of the hypersonic flow field through independently improved Roe entropy correction method and node Gaussian gradient algorithm, together with the wall criterion applied during the automatic hole-cutting procedure. The dual-time method was used to solve the unsteady separation process, and the LU-SGS algorithm was adopted for the internal iteration calculation. Firstly, the WPFS standard model was simulated to verify the accuracy of the software. Then the steady aerodynamic characteristics of the TSTO assembly were obtained and analyzed for the first and second stages at different angles of attack and possible safe separation states. Finally, the separation process of TSTO under a negative angle of attack and a positive sideslip angle was numerically simulated, and the time variations of the centroid displacement and attitude angle of the two stages were compared with the CTS experimental results, which showed a good agreement. The present study suggests that NNW-FlowStar can well simulate the interstage separation process of parallel aircraft.