When the straddle monorail vehicle passes through the curve, the guiding stiffness of the steering assist device(SAD for short) and the resistance stiffness of the secondary suspension cancel out each other, which is conducive to the bogie being in the radial position of the curve and reducing the tire wear. The linear model stiffness of the SAD was deduced, the guiding stiffness of SAD was tested when the vehicle passed through the curve, and the error of the linear model stiffness was analyzed. The output force characteristics of the oil-gas spring of SAD were analyzed, and a simplified calculation method of the initial output force of the oil-gas spring was proposed based on radial conditions, which can be used to direct the parameters design of the SAD. Finally, the influence of different initial output force of oil-gas spring on lateral ride comfort was given by dynamic simulation. The simulation results show that the lateral ride comfort is excellent, greatly influenced by vehicle speed and less by the adverse influence of initial output force of oil-gas spring.