Based on the differences of arteriovenous blood in the optical absorption spectrum, photoacoustic functional imaging obtains the spatial distribution of blood oxygen saturation in biological tissues by multi-wavelength photoacoustic excitation and detection, which provides important functional information for medical research and disease diagnosis. However, limited by the ultrasonic detection sensitivity of piezoelectric sensors, miniaturized photoacoustic imaging technology has a large error in the measurement of blood oxygen saturation, and the functional imaging capability is difficult to meet medical requirements. To address this problem, a distributed feedback Bragg fiber laser is considered as a sensitive element to detect weak photoacoustic signals, and the perturbation caused by ultrasound is read by the beat frequency between two orthogonal laser modes. The frequency noise of the laser is effectively suppressed by optical amplification, and thus the high-spatial-resolution functional imaging results of in vivo brain tissues and blood vessels in the rectum lining are obtained.