)1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, Xi'an 710119, China; [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (X.H. ); [EMAIL_ADDRESS] (W.N. )1 State Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese effectiveness and timeliness of hydroacoustic communication [4]. In addition, acoustic carriers are not suitable for use for air-water cross-media communication scenarios [5]. Underwater wireless optical communication (UWOC) systems, with high bandwidth, lightweight, and low power consumption, have an advantage over hydroacoustic communication in solving the problem of air-water high-speed transmission [6]. Blue-green light is usually used for underwater optical communication due to the relatively low light attenuation in seawater [7]. For UOWC, effects including seawater absorption, scattering, and turbulence make the optical signal attenuate severely during transmission, which limits the communication distance to two hundred meters [5,8]. In Table 1, we summarize the schemes and performance of published underwater wireless optical communication systems. The optical communication system equipped with a high-energy laser and high-sensitivity photon-counting model achieves the lowest received optical power and the highest total link loss [9]. However, limited by the repetition frequency of the pulsed laser, the communication rate of this optical communication system is only 12 kbps. In addition to the reception sensitivity of the communication system, the signal-to-noise ratio related to channel loss and environmental noise, detector noise, and detection schemes is another important factor. Due to the high link loss, after the optical signal is transmitted over a long distance in the underwater channel, the signal intensity drops to the level of a photon comparable to the receiver noise, which will cause the bit error rate to be too high to establish a reliable underwater optical communication link. Due to its outstanding noise suppression ability, the scheme of coherent detection has been widely used in free optical communication [10, 11, 12]. However, the fast decoherence of light in water limits the use of coherent optical communication for long-distance underwater communication. A potential way to improve the system's sensitivity against low SNR and high underwater channel loss is to directly make use of single photon detection and photon counting [13, 14, 15]. Here, we designed and demonstrated an underwater high-sensitivity wireless optical communication system combined with PPM and single photon counting techniques. The ultimate performance of the optical communication system was Evaluated by combining experimental results and Monte Carlo analysis tools. The following sections discuss the details of the theoretical analysis, experimental setup, and testing results.