The invention relates to a method for stably controlling the oxidability of ultra-low 
carbon steel top 
slag. The method comprises the following steps: S1, calculating the addition amounts of 
lime andan aluminum-based modifier according to the 
slag tapping amount and steel tapping amount of a converter, the TFe content in converter final 
slag and the 
oxygen content at the end point of the converter; S2, in the RH 
oxygen blowing temperature rising process, carrying out 
oxygen blowing and aluminum adding at the same time; S3, in the RH oxygen blowing 
decarburization process, controlling the oxygen blowing flow of a top lance to be 1,200-2,000 m<3> / h, so that the influence of sharp rising of oxygen in steel on the top slag is prevented; and S4, in the pouring process of a 
continuous casting machine, sealing a 
tundish, wherein the oxygen 
partial pressure in the 
tundish is controlled to be 0.3% or below. After the treatment measures are adopted, the content of TFe in the RH refining inlet steel 
ladle top slag can be stably reduced to 4% or below, the content of TFe in the RH refining outlet steel 
ladle top slag can be stably reduced to 6% or below, then the 
castability of 
molten steel is remarkably improved, the pouring service life of a single submerged 
nozzle can reach that of 6 furnaces or a longer time, and the total 
oxygen content of the 
tundish is reduced to 0.002% or below.