The purpose of this paper is to analyze the Flow field structure in Transient state and performance of secondary injection system for thrust vectoring in divergent section of a two-dimensional nozzle. Secondary injection for thrust vectoring in a two-dimensional nozzle is studied by solving threedimensional Reynolds-averaged equations by means of fluent solver. Spalart-Allmaras model was used to model the fluid behavior near the walls. Density-based solver and explicit formulation are employed in the computational model. Results show that the solution of interfered field in the Transient-state is more accurate than steady-state, especially in the initial injection. In addition, various testing showed that the maximum side force would be in the injection angle of twenty degrees and with increasing pressure ratio, we have more side force. At the end it was observed that by sketching the exit gases deviation according to time, we could be informed of desired secondary injection time in order to achieve required deviation around pitch axis, and the required force to achieve desired deviation angle. The innovation of this paper is the solution of interfered field in Transient state, and of course the injection from the optimal point.