Gully headcut retreat (GHR) is an important processes in gully development, and previous studies have tried to predict GHR rates based on flow hydraulic parameters during runoff scouring, but other dynamics such as gravity also affect GHR processes in addition to flow hydraulics. The aim of this study is to examine the influences of mass failures due to gravity on the erosion and hydraulics processes of gully headcuts during runoff scouring. A flow scouring experiment including 11 tests with two discharges (i.e., 7 tests with 83.3 l min−1 and 4 tests with 166.7 l min−1) was conducted on 5 in situ gully head plots with similar initial topographies. Totally 36 mass failure events were observed of all the 5 gully heads during the experiment, and 63.9% of the events were occurred during the first 10 min of the tests after drying-rewetting cycles. The tests were classified into two series according to whether the mass failure occurred during the scouring. The tests with mass failures (Series I) contributed 81.7% of the total soil loss of GHR, and the average GHR rates by mass (GHRm) was about 5 times higher than that of the tests without mass failures (Series II). The mass failure clearly influenced the flow resistances on gully beds by increasing the surfaces roughness and the sediment load to gully beds, and the average Darcy–Weisbach friction factor (f) and Manning coefficient (n) of Series I were clearly higher than those of Series II. Compared to the previous studies without significant soil collapse from gully heads, the logarithmic growth relationship between GHRm and the energy consumption (ΔEt) was quite weak in this study, but the R2 between GHRm and ΔEt increased for longer time scales. The results indicated that for individual runoff events, GHR rates could not be predicted by flow hydraulics alone because mass failures due to gravity made great contribution, but on longer time scales, the influence of soil collapse is reduced, and flow hydraulics become the key factor impacting GHR processes. •Mass failure is prone to occur at the beginning of scouring after drying-rewetting cycles.•Mass failure made great contribution on soil loss during headcut retreat.•The headcut retreat cannot be explained by hydraulics alone when mass failure occurs.•The influences of mass failure are reduced with longer timescales.