The invention provides a method for safely constructing a release hole outside a grouting curtain of a tunnel. The method comprises the following steps of: 1, designing a release hole according to the detected range of a concentrated water-rich area outside a curtain and the spatial position relationship between the range and a grout-preventing wall; 2, sequentially constructing primary, secondary and tertiary holes in the grouting curtain through the grout-preventing wall, and grouting and sealing primary and secondary casings; inserting release casings into the tertiary casing; mounting a release casing sealing connector with a stop valve on the release casing; 3, connecting a grouting connector which is connected with a double-liquid grouting mixer onto the secondary casing at the end part of the hole, and quantitatively grouting into the gap between the secondary casing and the release casing, stopping grouting when the slurry reaches the boundary of the concentrated water-rich area, closing the double-liquid grouting mixer, and waiting on cement setting; and 4, mounting a water-control valve in an opening state on the release casing at the end part of the hole so that water in the concentrated water-rich area can flow out from the water-control valve along the release casing.