Abstract: When a liquid bridge of a constant volume is formed between two surfaces, the width of a liquid-solid interface is constrained by the boundary parallel to the plate length direction and remains constant as the separation distance between those plates varies. However, the liquid bridge can expand or contract freely in length, which inevitably leads to the change in the shape parameters of the liquid bridge, thus changing the value of liquid bridge forces. According to the marginal effects of a slender liquid bridge profile, a pseudo-three-dimensional force model of a liquid bridge is developed. Then based on the characteristics of the slender structured surface and under the condition of a liquid bridge with a constant volume, an efficient method is employed to solve the differential equations directly. Finally, the relationship is explored between variable forces acting on the liquid bridge and variable shape parameters (as contact angle) and length and width with the variation of the separation distance (between parallel plates); meanwhile, the validity and practicality of the method are demonstrated by comparing the Surface Evolver (SE) simulations with those of the method employed in this paper.