Abstract: Aiming at the luffing process of a double-link gantry mechanism of a launch and recovery system, taking into account the gravity of the connecting rod, the driving speed and other factors, the hydraulic cylinder force was derived by the principle of moment equilibrium and the Lagrange kinetic equations respectively. The method of the moment equilibrium principle was classified, respectively considering the gravity of the connecting rod and not considering the gravity of the connecting rod. The derivation results of the three methods were taken into a specific model of the double-link luffing mechanism for verification. In addition, by using the Lagrange dynamic equation, the influence of driving speed on the force acting on the hydro-cylinder can be further analyzed. The results indicate that the results obtained by the three methods are not significantly different when the gantry is subjected to hydraulic cylinder thrust; during the tension process of the gantry, influence of the connecting rod weight and the overall angular velocity of the mechanism on force of the hydro-cylinder gradually increases, and the maximum tensile force calculated by the dynamic method is 473.8 kN, which is significantly greater than the 340 kN calculated by the moment equilibrium method. In the Lagrange dynamic model, the driving speed is increased by four times, and the deviation of the force curve of the oil cylinder is within 0.5%, which can be basically ignored in engineering applications. The above method can be extended to the theoretical calculation of other luffing mechanism, and provides a reference for the application of Lagrange dynamic equations and a theoretical basis for structural optimization design.