Relating to the technical field of refractory materials for coal chemical industry, the invention provides a preparation method and building method for a marked gasification furnace lower cinder notch. The method specifically includes the operation steps of: 1) material preparation: the marked gasification furnace lower cinder notch comprises, by mass, 79-90% of fused chromium oxide materials with a particle size of 5-0mm and 240 meshes respectively, 2-15% of an alumina micro-powder material with a particle size of 5-2 micrometers, 2-15% of chromium oxide green, 1-3% of a magnesium material with a particle size of less than 180 meshes, 1-3% of steel fiber and organic fiber, and 1-5% of a binding agent; 2) compounding; 3) premixing; and 4) material mixing, thus finishing preparation of the marked gasification furnace lower cinder notch. According to the invention, the magnesium material reacts with the acid binding agent and hardens rapidly, and when the temperature in the gasification furnace rises to certain degree, the magnesium material, chromium oxide and alumina form spinel so as to enhance the heat shock stability of the material, and the fireside surface refractory material of the lower cinder notch is easy to dismantle and replace.