As an evolutional concept of aero engine, the variable cycle engine (VCE) can adjust the thermodynamic cycle via working on different modes to meet different flight missions. These working modes lead to the performance and stability of variable geometries for mode switching under different mission profiles. This paper aims at the performance prediction under different working conditions on the key variable geometry, namely, the forward variable area bypass injector (FVABI). The rotary and translating mathematical calculation models are established and validated via the three-dimensional numerical simulation. It considers the choked flow state caused by area change at FVABI and mode selector valve (MSV). The variable geometry schedules at the typical subsonic cruise working condition are analyzed based on the zero-dimensional engine performance model. It verifies the efficiency of the improved zero-dimensional bypass mixing calculation model to predict the mixing characteristics of the FVABI on different operating modes. The maximum error of the total pressure recovery coefficient and the ejection ratio is 2.3% and 6.2%, respectively. This performance prediction method can lay foundations for the variable geometry design and performance optimization of the VCE under typical mission profiles.