Aiming at the difficulty of quantitative evaluation of onboard LOC receiving function in flight test of LOC receiving function of instrument landing system (ILS) for civil aircraft, this paper analyzes the basic working principle of LOC and its ground station signal, and combines the flight verification method and results of airport LOC ground station,and analyzes the error of runway LOC ground station itself and the error of calibration equipment is taken into account. Combined with the flight test method and flight test content of civil aircraft airborne LOC system flight test subject and based on the accuracy requirements of the relative system specification, an effective quantitative evaluation method for two different situations which are flying along the localizer oncourse and offcourse is presented. Furthermore, the actual flight test data has verified the correctness and scientificity of the method, which can support the current domestic qualitative assessment and can provide valuable reference for other similar system function quantitative evaluation.