The invention discloses an ultrasonic cutting method adopting a straight-blade sharp knife. The ultrasonic cutting method comprises the following specific steps of: S1, measuring parameters of the straight-blade sharp knife; S2, enabling the straight-blade sharp knife to initially rotate around a knife shaft of the straight-blade sharp knife, enabling a rear knife surface of the straight-blade sharp knife to fit to or be away from a processed surface, and performing ultrasonic vibration cutting on a material according to a processing track; S3, performing quality detection on the processed material surface obtained by ultrasonic vibration cutting, completing processing if detection is passed, and performing step S4 if detection is not passed; and S4, further increasing initial rotation amount, around the knife shaft, of the straight-blade sharp knife, performing ultrasonic vibration cutting on the material according to the processing track, and performing the step S3. The ultrasonic cutting method can effectively reduce extrusion, on the processed surface, of the rear knife surface of the straight-blade sharp knife, and improves surface processing quality of the material.