The effects of the optical properties of gold nanoparticle (AuNP) on the photoacoustic (PA) signals were investigated with the numerical simulation and experiment using various spherical and polyhedral AuNPs. In the numerical simulation, the absorption and scattering cross sections were calculated by the discrete dipole approximation. The absorbed light energy was calculated by the Monte Carlo method. Then PA pressure wave was simulated based on the PA wave equation. In the experiment, the PA signals from the AuNP suspensions were measured by use of a P(VDF-TrFE) piezoelectric film and a Q-switched Nd:YAG laser operated at 532 nm. It was demonstrated that the PA signals were characterized by both of the absorption cross sections and the scattering cross section. The scattering medium containing the AuNPs enhanced the amplitude of the PA signals.