Using the self-developed visualization test apparatus with constant slope and rainfall intensity and making six sets of slopes with different fine contents，centrifuge model tests on rainfall-induced debris flow were carried out.Through the analysis of slope failure patterns，the development of pore water pressure，moisture migration，and the influence of fine contents on the debris flow initiation process were studied.The test results indicate that there is a critical fine content（about15%）with the initiation of debris flow，the failure pattern of slopes is retrograded sliding when fine contents are blow the critical value，and the start time of debris flow with 10% fine content is the shortest.During the initiation of debris flow，the pore pressure increases to the peak before it declines like a wave and the curve of pore pressure tends to be gentle with the increase of fine contents.Fine contents influence the infiltration rate and the path of water，which affects slope failure patterns and determines whether the debris flow can start.