The size of the overlap rate affects the surface flatness of the multi-overlap laser cladding layer. In order to study the effect of overlap rate on the surface flatness of laser cladding on inclined substrates and obtain the critical overlap rate for flat surfaces, an elliptical model was used to fit the cross-section of the tilted substrate cladding layer, which was verified by experiments. The overlap model of the inclined substrate was established, and the calculation method of the critical lap rate to obtain the best flatness on the inclined substrate was derived. The effect of different overlap rates on the flatness of the clad layer is investigated at a substrate inclination of 30°. Results show that a smaller overlap rate leads to a decrease in the flatness of the clad layer and a larger overlap rate leads to an increase in the height of the clad layer due to the continuous accumulation of the clad layer, and that a flat clad layer can be obtained with almost no change in the height of the clad layer at the critical overlap rate.