Haermu ravine is located in the middle reaches of Zagunao river and belongs to “5·12” Wenchuan earthquake area, where debris flow rushed down many times in the history and once blocked off Zagunao river in August 1989. By the influence of “5·12” earthquake, the debris flow is with the characteristics of larger scale, higher occurrence frequency and longer active period than before. Based on the analysis on natural environment background of Haermu ravine, it is found that the topography and precipitation conditions of Haermu ravine are conducive to the outbreak of debris flow; by the influence of neotectonics movement and earthquake, large amount of slumped mass developed in ravine domain, which provides a wealth of loose solid material for debris flow. Currently, due to the enough loose solid material and the low rainfall threshold of debris flow, debris flow takes place every year in recent 5 years. According to the specific conditions of Haermu ravine, the disaster control idea of interception combined with drainage was proposed, blocking dam was built to regulate the peak flow of debris flow and drainage canal was utilized to drain the debris flow to the main river. Through nearly 5 hydrologic years, good effects of control engineering are presented.