Abstract: The flow and propulsion characteristics of a supersonic split line nozzle in over-expanded states are investigated numerically. The typical nozzle flow structures and operating modes are demonstrated based on a comprehensive analysis of flow patterns. Particularly, the inherent relationship between the operating modes and thrust vectoring performance is discussed in detail. The results show that the nozzle thrust vectoring performance in over-expanded states is determined by flow separation and geometric deflection in a combined way. Two typical operating modes, viz., the open mode and the closed mode, are identified according to the existence or absence of a large-scale separation zone downstream of the split line. In the open mode, the large-scale separation zone hinders the deflection of the supersonic exhaust flow and degrades the thrust vectoring performance. When entering the closed mode, the complex overall Mach reflection (oMR) wave system and separated flow in the nozzle shift to overall regular reflection (oRR) wave system and attached flow. Correspondingly, the nozzle lateral thrust increases monotonically with nozzle pressure ratio (NPR) increase, and the thrust vectoring efficiency stabilizes. Owing to the high sensitivity of the nozzle lateral thrust to the operating mode, the overall thrust vectoring performance exhibits a segmented varying pattern. Based on the conventional nozzle flow theory, the lateral thrust performance of the present supersonic split line nozzle is quantified and formulated by considering flow parameters at the nozzle exit. This work is expected to provide a physically clear evaluation for the thrust vectoring performance of supersonic split line nozzles.