The invention relates to a detection method for determining harmfulness of hydropower engineering environmental slope dangerous rock mass. The detection method can be used for determining the harmfulness of the hydropower engineering environmental slope dangerous rock mass more accurately. The detection method includes the following steps of a, determining classification objects of hydropower construction environment, b, performing evaluation at a construction site, c, determining the harmfulness of the hydropower engineering environmental slope dangerous rock mass according to site classification results determined in the step b, d, performing corresponding prevention measures on the dangerous rock mass according to harmfulness results, obtained in the step c, of the hydropower engineering environmental slope dangerous rock mass. According to the detection method, a win-win situation of safety protection and economic benefits can be achieved. The detection method can be used for determining the harmfulness of the hydropower engineering environmental slope dangerous rock mass and effectively improve construction protection measures for the harmfulness of the dangerous rock mass.