Flat plate structures are widely used in subsea equipment， and the water-entry slamming dynamics during installation operations have a significant influence on structural safety. In particular， the effect of entry angle has not yet been sufficiently investigated. In this study， a three-dimensional numerical model for flat plate water entry slamming was established based on the smoothed-particle hydrodynamics （SPH） method combined with an adaptive particle refinement technique. The influence of entry angle on the slamming dynamics of a flat plate during water entry was systematically analyzed. The results indicate that entry angle significantly alters the interaction mechanism between the flat plate and water. During vertical water entry （0° entry angle）， the blockage effect of water leads to a stronger concentration of energy， resulting in more violent slamming. In contrast， during oblique water entry， the lateral escape effect of water is enhanced， causing the slamming flow field to transform into an asymmetric structure and significantly reducing slamming loads. The slamming load exhibits a negative correlation with entry angle. At an entry angle of 5°，the pressure peaks at different measurement points show noticeable temporal discrepancies and are only about 70% of those under vertical entry conditions. Moreover， the slamming force at 5° is only approximately 20% of that at 0°.This study systematically investigated the influence of entry angle on slamming dynamics of flat plate water entry and can provide theoretical guidance for design and installation of subsea structures.