Signal attenuation is highly concerned in many research fields. Imaging quality and depth in photoacoustic (PA) imaging system are often limited by the attenuation of PA signal. The PA signal attenuation is induced by many factors in PA imaging system. The factors leading to the signal attenuation and its characteristics are discussed based on tissue optics, acoustic transport and detection in a long-focal-zone PA imaging system. Considering to recover the PA signal from optical to ultrasonic attenuation, a method to compensate the detected PA signals is presented. The proposed method is evaluated by simulant sample and employed to image a thyroid tissue in vitro. The experimental results demonstrate that the method could be used to recover the PA signal and to improve the imaging depth and quality in the PA system.