A check dam is one of the main control works for debris flow. The overflow condition under an empty library has been considered to be most dangerous for a long time in the stability calculation of the check dam. However, the action process between the debris flow and check dam is very complex, and determining the dangerous working conditions by a simple mechanical analysis about its stability calculation is not scientific. This paper comes to the stability formula by analyzing the actual load of the check dam according to the actual situation as closely as possible. Combining with a debris flow prevention engineering example of debris flow, Sabo dam, this paper illustrates the most dangerous conditions of a check dam. Keywords: debris flow, check dam, stability, dangerous condition.