The forming process of the photoacoustic signal excited by the intensity-modulated continuous-wave laser and the effects of modulation pulse width on photoacoustic signal are researched. It is shown that the power absorbed by the biological tissue increases with the increase of the width of the modulated rectangular pulse and the amplitude of the photoacoustic signal. The effects on the axial resolution of the photoacoustic signal excited by the modulated rectangular pulse are also researched. It is shown that the width of the photoacoustic signal increases with the increase of the width of the modulated rectangular pulse, and the axial resolution of the photoacoustic imaging becomes worse. The modulated rectangular pulse of the continuous-wave laser (laser diode) is used to induce the acoustic signal. The laser power density is raised to obtain photoacoustic imaging with higher signal-to-noise ratio (SNR) and resolution when it is hard to raise the laser power. The research on the influential factor of photoacoustic signal excited by intensity-modulated continuous-wave laser in biological tissue can be used to provide a portable and lower cost instrument.