Abstract: Due to the excellent comprehensive properties, 300M ultra-high strength steel is widely used in the manufacturing of various important forging parts in the aircraft landing gear systems. During the hot working production process, long term high temperature heating and heat treatment inevitably leads to the decarburization of the material, which significantly influences the microstructure and mechanical properties of the steel forging surface.It is a material defect that is not allowed to appear on aircraft landing gear parts. In order to study the surface decarburation behavior of 300M ultra-high strength steel, the effects of heating time, heating temperature and heat treatment process were investigated by isothermal heating experiments in a box-type resistance furnace based on the metallographic method. The results show that when the heating temperature is 1160 ℃, within the range of 0.5-2 h of holding time, the depth of decarburization layer of 300M ultra-high strength steel increases with the extension of holding time, however, the growth trend of decarburization layer depth slows down with the increase of holding time; when the holding time is 2 h, within the heating temperature range of 800-1200℃, the depth of the decarburization layer increases firstly and then decreases with the increase of heating temperature, the depth of the decarburization layer reaches maximum value at 1100 ℃; under different heat treatment processes, the surface decarburization depth of 300M ultra-high strength steel is the smallest without heat treatment, while the decarburization layer depth is the highest under the heat treatment process condition of normalizing/tempering+quenching/tempering.