The erosion shape and the law of development of debris flow sabo dam downstream is a weak part in the study on debris flow erosion. The shape and development of scour pit have an important effect on the stability and safety of debris flow sabo dam, which determines the foundational depth of the dam and the  design of protective measures downstream. Study  on the scouring law of sabo dam downstream can evaluate the erosion range and reasonably arrange auxiliary protective engineering. Therefore, a series of flume experiments are carried out including different debris flow characteristics (density is varying from 1.5 t/m3 to 2.1 t/m3) and different gully longitudinal slopes. The result shows that the scour pit appears as an oval shape in a plane and deep in the middle while superficial at the ends in the longitudinal section, the position of the maximum depth point moves towards downstream with an increase of flume slope angle. The maximum depth of scour pit is mainly affected by the longitudinal slope of gully, density of debris flow, and the characteristics of  gully composition (particle size and the viscosity of soil). The result also indicates that the viscosity of soil will weaken the erosion extent. The interior slopes of scour pit are different between the upstream and the  downstream, and the downstream slope is smaller than the upper one. For the viscous and non-viscous sands with the same distribution of gradation, the interior slope of nonviscous sand is smaller than the viscous sand. According to the regression analysis on the experimental data, the quantitativerelationship between the interior slope of scour pit, slope of repose under water and the longitudinal slope of gully is established and it can be used to calculate the interior slope of scour pit. The results can provide the basis for the parameter design of the debris flow control engineering foundation.