For the specific energy distribution of Gaussian laser, the obtained grooves always fall short in the sidewall verticality. To overcome this problem, the improvement of sidewall inclination in laser grooving of a CVD diamond is undertaken by the surface tilting motion control, where the insufficient material removal at the groove sidewall is relieved. Combined with finite element modelling, the influence of laser energy density, scanning speed, scanning times and scanning pitch on the inclination of sidewall are firstly analyzed, which shows that laser energy density is the main factor that dominates the sidewall inclination. The finite element simulation model (FEM) is established to achieve the contour evolution of the machining area under different tilted angles, where the generation mechanism of 'V' shape or 'W' shape at the bottom of the groove is analyzed. Finally, the inclination degree of the groove sidewall can be effectively controlled by optimizing the relative incident angle under the selected laser energy density.