The theory of photoacoustic (PA) imaging is introduced. The existing reconstrution methods, their foundmental and results at home and abroad are summarized. The filtered back-projection algorithm and the Fourier transform method based on acoustic lens, which can realize reconstruction of PA image from the PA signals recorded by ultrasonic sensors are both described. The demodulation of PA signals by optical systems is also introduced. In the optical methods, PA signals are modulated by probing optical beam and turned into optical signals, then it can be demodulated by different optical systems, and finally the sample image can be monitored by CCD camera dynamically. Comparison is made among the different real-time PA imaging methods and some new ideas are proposed.