Abstract: Abstract: The problem of lateral rollover due to excessive speed occurs easily in the process that the vehicle is turning, especially under the condition of vehicle extreme turn. In traditional researches on vehicle roller prevention, rollover threshold is calculated by vehicle tread, height of centroid and other structure parameters. The side acceleration, which is caused when vehicles are turning, is controlled on the basis of rollover thresholds to prevent rollover. However, sometimes the speed calculated by rollover threshold is very high. The lateral load transfer and suspension movement become larger because of sharp turn in actual turning, which can lead to rollover although the speed is under the calculated critical velocity. To solve the problem, the safety speed calculation model of rollover prevention is built, which is under the condition of vehicle extreme turn based on lateral force coefficient. First of all, combined with the expression of lateral force coefficient, and according to the influence of lateral force coefficient on the driving stability in the process of turning, the feasibility of characterizing the safety speed of the vehicle for rollover prevention under vehicle extreme turn condition by lateral force coefficient is analyzed. Secondly, the rigid body dynamic model of vehicle during the turning and the dynamic model with consideration of the suspension of vehicle are established. Through the comparison between the models above, the stress of the vehicle under the condition of extreme turn driving is analyzed. Thirdly, in view of the dynamic models above, the calculation method of traditional lateral force coefficient is optimized. Fourthly, according to the relationship between lateral force coefficient and vehicle speed, the calculation model of the safety speed of the vehicle based on lateral force coefficient is established, which is for rollover prevention under extreme turn driving conditions. Finally, to verify the accuracy and reliability of the model, taking the structure parameters of a truck as the example, through Matlab software, the relationship of turning radius and vehicle safety speed among the model and other 4 representative safety speed calculation models during turning is simulated and compared. It shows that the results from the speed calculation model based on lateral force coefficient are similar to those from other 4 modes, especially approximate with those from Lusetti model. Simultaneously, to verify the reliability of the built mode, the critical safety vehicle speed for rollover prevention in the process of vehicle turning with different turning radius is simulated by using the vehicle dynamics simulation software TruckSim. Comparing the rollover critical vehicle speeds from the simulation software and the speed calculation model based on lateral force coefficient, it shows that the critical safety vehicle speed from the method proposed in the paper is approximately 10% less than that from the TruckSim software. The results in the paper provide a reference for the calculation of safety vehicle speed, the stability evaluation and the active safety control of the vehicle under the condition of extreme turn.