To analyze the safety of elevator system quantitatively, the risk factor weights aiming at the risk evaluation for elevator system are analyzed based on Analytic Hierarchy Process (AHP) in this paper. The Accident Tree Analysis (ATA) is also introduced to analyze accidents leading to the dangers during elevator running as the basis of weight assignment. The risk evaluation model of elevator system is built and its safety grade is set. The quantitative assessment for elevator system through linear weighted sum law is implemented. This method combining AHP and ATA in this paper avoids subjectivity and one-sidedness of weights indexes caused by expert-scoring method. This elevator system risk evaluation system provides feasible method of assessing safety grade subjectively.