In this work, we aimed to study the austenite grain growth behavior of an ultra-high-strength stainless steel within the temperature range of 900-1150 °C and holding time range of 0-120 min, using a metallographic microscope and metallographic image analysis software to perform a statistical analysis of grain size variation. The undissolved phases of the steel were investigated using a field emission scanning electron microscope (SEM) and transmission electron microscope (TEM). Within the temperature range of 900-950 °C, the grain growth rate of the steel was slow, while within the range of 1000-1150 °C, the grain growth rate was relatively fast. This is attributed to the precipitation of a large number of M