During laser powder bed fusion (LPBF) process, grains undergo complex thermal-cycle stages. Driven by complex thermal loads, different grains undergo different degrees of dislocation multiplication and annihilation because of complex grain orientation and morphologies, leading to significant heterogeneous deformation and non-uniform residual stress distribution. On the other hand, under the multiple effects of complex thermal activation, dislocation evolution and deformation energy accumulation, various microstructural evolution, including liquid-solid (L/S) phase transformation, grain coarsening, dynamic recrystallisation (DRX) and dynamic recovery, can be activated in different stage. Notably, the interdependence of multiple microstructure evolution at different stages complicates the grain morphology and dislocation evolution, and it in turn influences the mechanical response and the corresponding polycrystalline deformation, exhibiting a complex two-way coupling mechanism, which makes tracking the thermal-mechanical behaviour more challenging. Therefore, a full-stage fully coupled phase field-crystal plasticity finite element model (PF-CPFEM) is developed to consider the interactive mechanisms of heterogenous deformation, polycrystalline mechanical response, and microstructure evolution during LPBF process. In the model, a multi-physic thermal-fluid flow model is adopted to obtain the shape and temperature field of the molten pool. Subsequently, a PF model of L/S phase transformation, grain coarsening and DRX with unified order parameters and governing equations is developed and then fully coupled into a dislocation-based CPFEM as an intrinsic part of the thermal-mechanical constitutive. Based on this model, different phenomena including DRX and L/S transformation in LBPF are predicted. The correlation between the microstructure and residual stress is also studied.