As one of the fastest-growing imaging modalities in recent years, photoacoustic imaging has attracted tremendous research interest for various applications including anatomical, functional and molecular imaging. Majority of the photoacoustic imaging systems are based on time-domain pulsed photoacoustic method, which utilizes pulsed laser source to induce wideband photoacoustic signal revealing optical absorption contrast. An alternative way is frequency-domain photoacoustic method utilizing chirping modulation of laser intensity to achieve lower system cost. In this paper, we report another way of photoacoustic method, called phase-domain photoacoustic sensing, which explores the phase difference between two consequent intensity-modulated laser pulses induced photoacoustic measurements to reveal the optical property. The basic principle is introduced, modelled and experimentally validated in this paper, which opens another potential pathway to perform photoacoustic sensing and imaging eliminating acoustic detection variations beyond the conventional time-domain and frequency-domain photoacoustic methods.