The invention relates to a method for assembling gasification furnace internal gas-pipe line 45-degree water screen cylinder body, which is characterized in that a locating rack of the gasification furnace internal gas-pipe line 45-degree water screen cylinder body is manufactured and is positioned by a central locating pipe, a circular arc locating device and a circular arc belt comb shaped locating device, and then three pipes are sequentially arranged on the locating rack of the gasification furnace internal gas-pipe line 45-degree water screen cylinder body to form a round rack pipe to bewelded. The invention has the advantages of solving the manufacturing problem of the gasification furnace 45-degree water screen cylinder body, ensuring the dimensional requirements of the product and better guaranteeing the manufacture of a gasification furnace.