To investigate a theoretical algorithm model of the lateral stability of tank trucks and its relationship with the liquid filling rate, the shifting of the liquid centroid during non-full loading is considered. The deformation of the suspension and tires under load is assumed to exhibit a linear relationship. Using the principle of moment balance, a calculation model for the maximum stable roll angle and rollover critical acceleration of tank trucks based on the “liquid–tank–vehicle” tri-coupling is established, and the relationship between the maximum stable roll angle and rollover critical acceleration of tank trucks is analyzed and compared. Based on the result and a specific tank truck model, the effects of the liquid filling rate on the maximum stable roll angle, rollover critical acceleration, and tank tilt angle of tank trucks are investigated using numerical analysis methods. A comparison with the lateral stability of ordinary trucks loaded with solid goods is performed to further examine the effect of the liquid filling rate on the lateral stability of tank trucks. The results show that the maximum stable roll angle and rollover critical acceleration are consistent indicators for evaluating the lateral stability of tank trucks; the lateral stability of tank trucks decreases owing to the shift in the liquid centroid inside the tank; as the liquid filling rate increases, the lateral stability of the tank trucks deteriorates slightly.