The invention discloses a positioning device and method for a tunnel oblique hole slotting blast hole. The device comprises a blast hole positioning mechanism which is attached to the tunnel face of atunnel, and the blast hole positioning mechanism is connected with a blast hole angle adjusting mechanism located on the outer side of the tunnel face of the tunnel through a connecting mechanism; ablast hole positioning ring is arranged on the blast hole positioning mechanism, and the circle center of the blast hole positioning ring coincides with a blast hole identification point; an angle adjusting ring is arranged on the blast hole angle adjusting mechanism, and the circle center of the angle adjusting ring and the circle center of the blast hole locating ring are located on the same horizontal plane; and the angle adjusting ring is used for moving relative to the blast hole positioning ring on the same horizontal plane so as to adjust the inclined angle of a drill rod penetrating through the blast hole positioning ring and the angle adjusting ring. According to the device and the method, the inclined angles of slotting blast holes can be ensured to meet the design requirements,the bottoms of the plurality of slotting blast holes can be arranged on one vertical surface, so that the preset slotting effect can be achieved, and meanwhile, the device can be repeatedly used and can be applied to the condition of multiple slotting angles.