Based on acousto-optic refraction, a non-invasive method used for detecting acoustic pressure of the focus of focused ultrasound is proposed. By studying the specific relationship between acoustic pressure of the focus and the ray deflection when a parallel beam whose diameter is less than the wavelength of the sound wave is crossing the focus of the focused ultrasound, a relational model between the maximum deflection distance of the ray and the change of the focus acoustic pressure is established, thus the peak acoustic pressure of the focus is calculated. In order to verify the theoretical model, the concave sphere type focused transducers are used for experiment research. By comparing with the result measured by optical fiber hydrophone, the feasibility of the theoretical model is proved. The results show that the facula images acquired in experiment are consistent with the results of theoretical analysis, and the relative error between the acoustic pressure of the focus measured by current method and that by using optical fiber hydrophone is less than 15%, which proves the feasibility of the method. And it can be used to detect the peak acoustic pressure of the focus quantificationally. The proposition of this model provides the experimental and theoretical evidence for that the acousto-optic refraction effect can be used to measure the whole focused ultrasound field quantificationally.