The effects of ultrasonic treatment (UST) on the solidification microstructure of a 35CrMo steel cast ingot were investigated. To avoid the erosion of a high-temperature melt, a T-shaped ultrasonic waveguide unit was used to introduce ultrasonic vibrations into a 35CrMo steel die casting melt. The experimental results show that the microstructure of the ingot was refined by ultrasonic treatment. While the microstructure of untreated 35CrMo steel is coarse dendritic grains, after the introduction of ultrasonic treatment, the solidification microstructure transforms from coarse dendritic to equiaxed grains, and the dendrites are also refined. Samples from different positions of the ingots were subjected to different ultrasonic effects, and the effects on grain refinement also varied due to the severe attenuation of the ultrasound in the melt. The mechanisms of grain refinement using ultrasonic treatment for 35CrMo steel melt are presented.