ISO 26262 standard proposes to ensure the safety of the whole life cycle of vehicles by assessment the Automotive Safety Integrity Level (ASIL). As one of the assessment indicators of the ASIL, the assessment of controllability lacks a quantitative and systematic method. This paper provides a controllability metrics method for functional safety of adaptive cruise control (ACC) system. Firstly, a Carsim/Simulink co-simulation model is constructed based on the hazard events in the HARA analysis of ACC system. And then an improved Analytic Hierarchy Process (AHP) is adopted to constructed a controllability assessment model by combining quantitative and qualitative methods. The metrics of controllability is transformed into a comprehensive assessment of multiple indicators, and the weight of each controllability metric is obtained. Finally, the controllability comprehensive score is calculated by taking three operating conditions under high speed as an example.