Facing gears are widely used in aero-engine and helicopter equipment in the form of cross-shaft or intersecting-axis transmission. However, its unique face structure makes it difficult and inefficient to machine, and offline inspection leads to micrometrescale machining accuracy that is difficult to converge quickly. To address this problem, this article develops an on-machine measurement system for face gears. To solve the problems of low reconstruction accuracy of tooth face and difficulty in constructing and slow solving coordinate system in the on-machine measurement system, a mathematical model of tooth face is formulated, which is based on the principle of gear insertion processing. The evaluation method of tooth face error with segmentation approximation and the fast solution method of measurement coordinate system with normal distance constraints are proposed. The accurate solution of tooth surface error and the fast solution of measuring coordinate system are realized. Finally, the on-machine measurement of face gears is completed on the YK7280 CNC grinding machine to evaluate the accuracy of the proposed algorithms as well as to validate the accuracy of the system with the Klinberg Gear Measuring Center. The experiments show that the developed on-machine measurement system has a tooth shape error of 6 μm, which meets the machining and inspection requirements of grade 5 face gears.