The invention relates to a liquid-state lag tapping device of a slack coal slag gasifier. The liquid-state lag tapping device comprises a main body, a cooling agent flowing pipeline and a slag tapping channel, wherein the cooling agent flowing pipeline is buried in the main body; the slag tapping channel penetrates through the main body, extends to the bottom end of the main body from the top end of the main body, and is symmetrically arranged around the longitudinal axis of the main body; and through the slag tapping channel, the upside of the liquid-state slag tapping device is communicated with a coal gasification pressure container and the downside of the liquid-state slag tapping device is communicated with a chilling chamber; the main body comprises a main body upper part and a main body lower part, the slag tapping channel limited by the inner surface of the main body upper part is downward converged along with the longitudinal axis of the main body; and the slag tap of the liquid-state slag tapping device is limited by the inner surface of the main body lower part. The liquid-state lag tapping device for the slack coal slag gasifier can avoid or withstand the invasion and corrosion of molten slag and ferrum as well as thermal impact influence, and is long in service life and convenient to disassemble and maintain.