The invention provides a 90-ton basic oxygen furnace bottom maintenance method. The carbon content of steel tapping is greater than 0.07wt%. After the steel tapping, slag splashing is normally carried out, and the furnace has slag left of 1 to 2 tons after the slag splashing. The relative lance position of an oxygen lance is set at 900-1100mm, and the oxygen pressure and the flow are controlled. After firing, the relative oxygen lance position is reduced to 700mm, and the oxygen lance position is automatically lifted up after one minute oxygen-blowing. A slag-adjusting agent is added and after cooling down, the slag is removed. The furnace bottom maintenance method of the invention has the advantages of simplicity and easy implementation, and convenient operation and control, reduces the furnace bottom thickness by 20 to 25mm so as to keep a liquid level in the furnace molten pool unchanged, has a stable converting process, greatly reduces slag overflow during the converting process, and ensures the production safety. Compared with the prior art, the furnace bottom maintenance method of the invention greatly saves the cost and resources, reduces the temperature drop, and shortens the smelting time.