In order to reduce the risk of rollover accident in the process of liquid tank semi-trailer driving through a curved road section, a control method for the safe speed of liquid tank semi-trailer on curved road was proposed on the basis of optimal control theory. This control method prioritizes the prevention and control of rollover accidents before traffic efficiency, and achieves "preventive" active safety control by providing optimal speed control profiles for liquid tank semi-trailers driving towards curved road, without potential lateral instability hazards. The sine wave equation was used to fit the longitudinal transfer process of liquid in the tank to improve the effectiveness of the control profiles. Then a tank liquid sloshing model was built to describe the liquid motion on both vertical and horizontal dimensions. This model utilizes the lateral displacement law of liquid in the tank based on the improved quasi-static method, which can improve the accuracy of the lateral steady state characterization index of liquid tank semi-trailer in the cost function of the optimal control problem. The simulation results show as follows. The instantaneous lateral load transfer rates of the controlled liquid tank semi-trailers during driving through curved roads are within the safe range of [-0.7, 0.7]. The decreasing of instantaneous lateral load transfer rate of vehicles could be observed under different curve radii, and the maximum decline occurs at curve radius of 61 meters. The maximum instantaneous lateral load transfer rate of the controlled vehicle has a maximum reduction of 0.07. The control profiles have less negative impact on traffic efficiency. It is concluded that this optimization control method of safety vehicle speed helps liquid tank semi-trailers avoid rollover accidents, and is suitable for various curved road sections with different radii.