Abstract: Modern spacecraft usually carry a large amount of liquid propellant. The consumption of propellant leads to the change of the equivalent model parameters used for propellant sloshing modeling. In order to obtain the accurate equivalent model parameters and introduce them into the closed-loop control of GNC system to improve the attitude control accuracy of spacecraft, this paper proposes an on-orbit identification strategy for equivalent model parameters based on Square Root Cubature Kalman filter (SR-CKF). Firstly, in order to establish an equivalent model applicable to the metal diaphragm tank, a torsion spring-damper is applied at the suspension point of the equivalent spherical pendulum model to equate the stiffness-damping effect of the metal diaphragm on the propellant, and the spacecraft rigid-liquid coupled dynamic equations are established with the help of Kane's method. Secondly, a SR-CKF based on-orbit identification strategy of the equivalent model parameters is proposed for the situation where the equivalent model parameters are unknown in the state feedback backstepping controller. This strategy can identify the parameters of the equivalent model and the remaining amount of propellant in the tank and predict the propellant distribution motion state online according to the on-board angular velocity sensor data while the spacecraft completes a large-angle attitude maneuver mission. Finally, the numerical simulation results demonstrate the effectiveness and necessity of the on-orbit identification strategy proposed in this paper. This work has important reference value for the calibration iteration of the equivalent model in the GNC system of spacecraft, the study of the on-orbit sloshing behavior of propellants, and the prediction of service life of spacecraft.