In order to further grasp the subsurface damage mechanism of ultrasonic vibration-assisted grinding of optical glass materials, the Vickers indentation experiment under non-ultrasonic vibration and ultrasonic vibration conditions was designed to investigate the indentation features of K9 optical glass under two conditions. The magnetic component fluid polishing was used to detect the depth of the median crack in the indentation area of K9 optical glass. The conventional model of median crack of indentation was modified twice to obtain the model of the Vickers median crack depth of the indentation under ultrasonic vibration conditions. The static and dynamic fracture toughness were calculated by experimental data of Vickers indentation under non-ultrasonic vibration and ultrasonic vibration conditions. The first correction coefficients of the two conditions were 0.08 and 0.06 respectively. Combined with the experimental results of the measured median crack depth, the values of the second correction coefficients under the two conditions were close under two conditions, which were 94.75 and 94.50 respectively. The results show that the new median crack depth model has a good recognition of ultrasonic vibration and processing conditions.