Photoacoustic signals induced by a short pulse laser cover a wide spectral range. The spectral characteristics of absorbers with different sizes and the influence of photoacoustic signals with different spectral components on photoacoustic imaging are explored. The main frequency ranges of photoacoustic pressures of absorbers with diameters of ～cm, ～mm and hundreds of μm are about 20～300 kHz, 70 kHz～2.5 MHz and 400 kHz～20 MHz, respectively. The low spectral components of photoacoustic signals contributed to the non-boundary region of absorbers, and the high spectral components contributed to the small structure, especially, to the boundary. It is demonstrated that the ultrasonic transducers used to detect photoacoustic pressures should be designed and selected according to the frequency ranges of absorbers. When the frequency response range of transducers accords to that of absorbers, almost the whole frequency components of photoacoustic pressure can be detected. That produces good reconstructed images. A Q-switched Nd∶YAG laser operating at 532 nm was used as light source. The laser had a pulse width of 7 ns and a repetition frequency of 30 Hz, and a needle PVDF hydrophone with diameter of 1 mm was used to detect photoacoustic signals.