Abstract: Abstract: In view of the limitations of the existing underground wireless communication system in coal mines in terms of disaster resistance, anti-interference ability, transmission distance, etc., this paper studies the communication technology of underground free space laser communication (FSO) in coal mines and proposes an underground emergency communication system based on it. Firstly, the channel model of downhole laser line-of-sight transmission is analyzed, and the analytical expressions of the received optical power and maximum communication distance are derived based on the theory of geometric diffusion and dielectric extinction, and the influence of key parameters such as extinction coefficient and receiving aperture on the communication distance is numerically analyzed. The results show that in the general environment of the underground roadway (extinction coefficient 0.01 m?1), the communication distance of a single hop can reach nearly 1 kilometer. In order to break through the bottleneck of hopping numbers caused by the relay delay of wireless communication, this paper innovatively proposes a microsecond laser transparent relay method based on "light-electricity-light" direct forwarding, and on this basis, a complete downhole laser emergency communication system is designed, and the design scheme of its network architecture, node equipment (downlink/uplink/cascade device) and core hardware circuit (laser drive, APD reception and amplification) is elaborated. Finally, the system performance is verified by experiments: the single-hop link realizes the stable transmission of signals in the roadway environment, and the rate remains unchanged after the introduction of transparent relay. This study provides an effective technical solution for underground emergency communication with high bandwidth, low latency and anti-interference.