Velocity of blood flow is measured by photoacoustic correlation spectroscopy. The effects of the laser repetition frequency and the angle between the blood flow direction and incidence laser propagation direction on the measurement of velocity of blood flow are studied. Research results show that the faster the blood flow velocity, the higher the laser repetition frequency required. When the blood flow orientation is perpendicular to the direction of incident laser propagation direction, the range of velocity of blood flow that the system can measure is 0.059-92.3 mm/s. The correlation coefficient between the measured flow velocities and the actual flow velocities is 0.992. When the blood flow orientation is not perpendicular to the direction of incident laser propagation direction, the ratios of the measured velocities of blood flow to the original ones are cosine to the tilted angle of sample.