[Objective] Headcut bed plunge pool point development is a key process in headcut erosion on granitic red soil slopes, but the mechanism of its formation is not clear. [Methods] We investigated the process of gully bed plunge pool development on granite red soil slopes by setting five headcut heights (25, 50, 75, 100 and 125 cm). [Results] (1) As the height of the headcut increased, the Re, τ, ω and the jet parameters (V_bottom, τ_max, EK_bottom, and ΔE_H) showed an increasing trend. (2) After the laterite soil layer scouring to form a "V" type of small plunge pool, with the increase in the flow of the form of plunge pool gradually increased, but it is difficult to form a large area of the plunge pool, and the laterite soil layer is more stable than the sandy soil layer. During the scouring process of the sandy soil layer, the development of headcut bed drop points was more obvious, and when the flow rate increased to 120 L/min, the degree of gully bed fragmentation was gradually drastic, and the outline of the drop points was gradually clearer, and the cross-sectional area increased. (3) The random forest algorithm and Shapley values were used to construct the model and characterize the relationship between the parameters. The results showed that in the laterite layer, Fr, ω, EK_brink, and Re were negatively correlated with the cross-sectional area; and in the sandy soil layer, Fr and ΔE_H were negatively correlated with the cross-sectional area. Higher accuracy of predictive models constructed by the random forest algorithm (LCCC=1.02, R²_adjusted=0.876 and 0.868). [Conclusion] The results of the study can provide a theoretical basis for revealing the mechanisms of headcut erosion and gully erosion control.