In order to avoid the sideslip or rollover under extreme turn conditions of bus with fast turning or too fast changing,first,a formula to calculate the lateral force coefficient considering the suspension movement effect is deduced by using the lateral force coefficient as an evaluation index of cornering stability under extreme turn conditions.Next,by analyzing the influence of the lateral force coefficient on the driving stability,the early warning threshold and the risk threshold of the lateral force coefficient under extreme turn conditions are both defined,and the relationship between the two thresholds and the vehicle speed is reveled.Then,in order to implement the active safety control of steering stability,a turning radius calculation model of cornering stability based on the lateral force coefficient threshold under extreme turn conditions is put forward.The proposed calculation model is finally verified through a comparative analysis by selecting some typical bus parameters.The results show that the calculation model is correct and effective,and that it helps to obtain a turning radius with a safety factor of 1. 15 to 1. 3,which is close to the limit value obtained by the simulation software IPG TruckMaker.This research provides a theoretical foundation for the cornering stability evaluation and active safety control of bus under extreme turn conditions.