The invention relates to a method for building a tunnel water taking opening or a water outlet in the bottom of a reservoir, in particular to an underwater rock plug blasting construction method. The method is achieved through the following technical schemes that the underwater rock plug blasting construction method includes the steps of 1, surveying and setting up, 2, selecting a rock plug opening and the position of a slag gathering pit, 3, processing a rock plug opening covering layer, 4, detecting the thickness of a rock plug body, 5, conducting geological sketch on the rock plug body cross section, rock plug body surrounding rock leakage analysis and rock plug body leaked water processing, 6, calculating the rock plug body safety thickness, 7, designing and arranging a rock plug body blasting scheme and 8, conducting blasting. By means of the method, it is avoided that a cofferdam is built in deep water, and the method is economical and safe.