Equivalent mechanical model can be used to describe liquid sloshing of a liquid-filled vehicle with small amplitude sloshing. However， in the study of effect of liquids on rigid body， modeling from the perspective of frequency domain is still absent. In this paper， based on the known equivalent mechanical models associated with various modes of liquid， the transfer function of longitudinal mode rudder deflection angle to attitude is given by the principle of virtual power and the principle of linear small disturbance. According to the concept of dominant pole， the high-order system of liquid-filled vehicle is approximated by typical transfer functions of second-order systems. The results show that the liquid mass and moment of inertia mainly affect the transfer coefficient and time constant of the vehicle， which reduce the maneuverability and the natural frequency of longitudinal mode of the vehicle， while the liquid sloshing mainly affects the relative damping coefficient in the typical transfer function， which reduces the longitudinal relative stability of the vehicle.