The invention provides austenite grain refined medium carbon steel and a preparation method thereof and belongs to the technical field of metal materials. The preparation method comprises the following steps of conducting high-pressure heat treatment on medium carbon steel and cooling the medium carbon steel by maintaining the high-pressure heat treatment pressure to obtain the austenite grain refined medium carbon steel, wherein the high-pressure heat treatment pressure is 1-3 GPa, the high-pressure heat treatment temperature is 900-1000 DEG C, and the high-pressure heat treatment time is 15-20 min. According to the preparation method, the medium carbon steel is subjected to high-pressure heat treatment; high pressure can cause high strain in the structure of the medium carbon steel to generate high-density dislocations; the dislocations provide more parts for crystal nuclei of austenite grains, and the quantity of the formed austenite crystal nuclei is increased. Besides, under highpressure, atom diffusion is difficult, and growth of the austenite crystal nuclei is inhibited. Thus, according to the technical scheme provided by the invention, the obtained austenite crystal nucleiare fine and uniform in distribution. Moreover, the preparation method provided by the invention is simple and short in process cycle, and technological parameters are easy to control.