In order to further reveal the mechanism of ultrasonic vibration assisted grinding, a prediction model of the ultrasonic vibration assisted grinding subsurface damage depth and fracture toughness was established. The single random-shape diamond grit indentation experiments without ultrasonic vibration and ultrasonic vibration assisted grinding experiments were designed. The indentation features of K9 optical glass in the two cases were investigated. A calculation method of equivalent fracture toughness suitable for two cases was proposed. The reliability of the model is verified by ultrasonic vibration assisted grinding experiments. The experimental results show that ultrasonic vibration can effectively increase the resistance of K9 optical glass to fracture, and the prediction model has a good consistency with the test results.