The ultra-high molecular weight polyethylene (UHMWPE) laminate has great application potential in lightweight body armor. However, due to the complex elastic-plastic dynamic response of UHMWPE laminate under ballistic impact, it is still challenging to characterize and predict, especially theoretically, the backface deformation (the apex displacement and the location of the boundary of the deformation region) of UHMWPE laminate, which is crucial for evaluating its protection performance. In this paper, the backface deformation of UHMWPE laminate is systematically investigated through numerical and theoretical analysis. First, an orthotropic elastic-plastic damage model considering the strain rate effect is established and implemented by user-defined subroutine VUMAT in ABAQUS, which can describe the dynamic response of UHMWPE laminates in agreement with the experimental results. Secondly, the numerical results show that the apex displacement of the UHMWPE laminate increases linearly with the increase of projectile impact velocity but decreases nonlinearly with the increase of the thickness of UHMWPE laminates. Besides, it is found that when the deformation of UHMWPE laminates is stable, the boundary of the deformation region (traveling hinge) expands outward at a constant speed, independent of the thickness of the UHMWPE laminate and projectile impact velocity. Finally, a theoretical model for the backface deformation prediction is derived based on the momentum conservation and the numerical analysis results. The proposed prediction model is well compared with the numerical results and can be used to predict the effects of the mass and the impact velocity of the projectile and the UHMWPE laminate thickness on the backface deformation. The theoretical analysis also reveals that the constant speed of the traveling hinge which is independent of the thickness of the UHMWPE laminate and impact velocity is attributed to the membrane stretching dominated deformation. The work in this paper is expected to help understand the dynamic response of UHMWPE laminates under projectile impact.