Ex-ante and ex-post assessments are both essential in risk management, although ex-post assessments of engineering measures have not been widely reported. In this study, an ex-post benefit-cost assessment method was developed to enable the direct impact of remediation engineering measures for debris flows to be quantified. We propose three categories of parameters for calculating benefit and cost: (1) predicted benefits (comprising predicted benefits of saving lives, at the household level, and of infrastructure protection); (2) actual loss (of losing lives, at the household level, and of infrastructure); (3) actual cost of the engineering. Actual benefit was defined as predicted benefit minus loss after completion of the engineering measure. The engineering measures were regarded as effective when benefits outweighed costs. Government data, survey results based on past disasters, and interview results with specialists were used. This methodology was applied and tested in ten villages located in the Gaoping River Basin in southern Taiwan. The result showed that the range of the benefit and cost ratio was between 1.26 and 7.99. We recommend this ex-post assessment method for rapid screening of districts with low benefit-cost ratios.