The phenomenon that vector angle of shock vector control nozzle decreases with increasing nozzle pressure ratio (NPR) has been verified by numerous researches. The study was involved with the mechanism of vector angle variation with NPR and multi-slot cavity and multi-slot assistant injection for separation control. A way for improvement of vector performance at large NPR conditions was studied. Results indicate that

the mechanism of vector angle variation with NPR is mainly due to the transition of separation region structure (from open to closed)

which is located downstream the secondary injection. This leads to the reduction of thrust vector force caused by wall pressure difference. With the help of multi-slot assistant injection

the separation can be kept open at large NPR conditions. Vector performance can be improved by atmosphere pressure injection without need of bleeding air from engine.