To address the inherent challenges in the precision machining of single-crystal diamond, which arises from its intrinsic properties of extreme hardness, high brittleness, and elevated melting point, a water-jet guided laser(WJGL) processing technique was proposed. with a comprehensive examination of the ablation dynamics of single-crystal diamond under this method. The nanosecond laser employed in WJGL processing serves as the primary energy source, with material ablation governed by the combined thermal effects of laser irradiation and the cooling influence of high-velocity water jet flushing. Underpinned by the first law of thermodynamics and the principles of jet cooling, a photothermal model was established to capture the laser ablation behavior of single-crystal diamond in the WJGL context. Detailed investigations were conducted into the temporal evolution of surface temperature under single-pulse conditions, as well as into the morphological changes in ablation patterns under multi-pulse exposure. The validity of the theoretical simulations was confirmed through single-point impact and linear scribing experiments. Comparative analysis of groove morphologies created by single laser versus WJGL scribing further elucidates the unique characteristics and material removal mechanisms inherent to WJGL processing of single-crystal diamond.