The most common method to measure an ultrasound pressure field, hydrophone scanning, requires a long scanning time and potentially disturbs the field, This may limit the efficiency in development of ultrasonic application in medicine. In this study, an optical phase contrast method, enabling the fast and noninterfering measurement, is proposed. This method measures the modulated phase of light caused by the focused ultrasound pulse pressure field. Then, a CT algorithm to quantitatively reconstruct the 3D pressure field is applied. The result agreed well with the measurement by a hydrophone.