The present invention relates to a thinner composition for removing resist used in TFT-LCD manufacturing processes, and more particularly to a thinner composition for removing resist that comprises: a) 0.1 to 5 wt % of an inorganic alkali compound; b) 0.1 to 5 wt % of an organic amine; c) 0.1 to 30 wt % of an organic solvent; d) 0.01 to 5 wt % of a surfactant comprising an ionic surfactant and a non-ionic surfactant in the weight ration of 1:5 to 1:25; and e) 60 to 99 wt % of water. The thinner composition for removing resist of the present invention has good efficiency of removing unwanted resist film constituents formed on the edge of the resist film or at the back of the substrate in TFT-LCD device and semiconductor device manufacturing processes. Also, it does not have the problem of equipment corrosion.