We theoretically prove that the point spread function (PSF) of a photoacoustic imaging system is the first derivative of the edge response function, and a step method for measurement of the photoacoustic PSF is proposed. The proposed method is experimentally verified using dyed agarose gel phantom, and the results of theoretical calculations and experimental measurement are in good agreement. The method provides a simple and practical way to measure the PSF of a complex photoacoustic imaging system.