As space exploration missions put higher requirements on spacecraft, new spacecraft often carry liquid fuels with large mass proportions, while a large number of flexible materials are widely used in spacecraft design and construction. Presently, the exploration of the dynamics and control of such spacecraft in aerospace engineering remains confined to discrete models of flexible appendages and equivalent mechanical representations of liquid sloshing. Challenges such as spillover effects and the nonlinearity inherent in liquid sloshing pose formidable obstacles, impeding the depth and comprehensiveness of research into in-orbit control. In this paper, the smooth particle hydrodynamics (SPH) method and the distributed parameter method are used to model and analyse the liquid-filled flexible spacecraft with respect to the complex coupling characteristics. Firstly, the sloshing part of the liquid is modeled by the SPH method, and the liquid sloshing force and torque are calculated by the SPH method in a non-inertial system; secondly, Then, a liquid-filled flexible spacecraft rigid-flexible-liquid coupled dynamics model was established by adopting Hamilton's principle and the distributed parameter method, balancing the computational steps between the SPH method and the spacecraft dynamics model, and integrating the liquid sloshing model into the rigid-flexible coupled model of the flexible spacecraft. Based on this model, two cases of symmetric vibration and antisymmetric vibration are designed for numerical simulation, and the coupling relationship between rigid-flexible-liquid is analysed in comparison with the rigid-flexible coupling model. The simulation results show that the liquid sloshing absorbs the vibration energy of the flexible appendages and excites the oscillation of the rigid body, which makes the rigid-flexible coupling model no longer satisfy the symmetric vibration or anti-symmetric vibration characteristics.