The construction of nanoprobes with high conversion efficiency is the key factor for the development of photoacoustic (PA) molecular imaging. The traditional design methods for PA probes usually maximize their optical absorption at the tissue optical window, while the research on the PA conversion properties of the probes has not been given enough attention. Taking gold nanospheres as an example, the mechanism of microcosmic PA conversion of nanoprobes in PA effect mediated by thermal expansion mechanism is discussed, which makes the rational design of probe with high conversion efficiency possible. Based on theoretical analysis and finite element analysis, it is indicated that nanoprobes no longer satisfy the thermal confinement conditions due to the small size effect. The thermal energy diffuses from the sphere to the surrounding medium in the range of laser pulse. As a result, the PA signal is composed of nanoprobes itself and the thermal expansion of media around the nanoprobes.