In atmospheric channel laser transmissions, atmospheric turbulence has a large influence on system performance, reducing its transmission rate and increasing its bit error rate. In a bidirectional free-space laser transmission link with channel reciprocity, as the change in optical signal intensity at the two terminals is correlated, the Channel State Information (CSI) can be obtained at the transmitter and used to compensate the channel influence, thus improves the transmission rate. In this paper, under weakly fluctuating conditions, according to Rytov approximation, the relationship between the correlation coefficient of the spot signal received by plane wave bidirectional transmission link and the transmission path is deduced, and its analytical expression is derived. The results show that the intensity of the optical signal at the receiving end of the bidirectional free-space laser transmission link is related to the transmission end and that the correlation coefficient is related to the location of the transmission path. A bidirectional coaxial laser transmission system is further established and an external field test is performed. The real-time change trend of the intensity of the speckle signal at both receivers is the same. Therefore, the atmospheric channel of the bidirectional free-space laser transmission link is reciprocal. The conclusion of this paper is of great significance for realizing high-rate and low bit error rate transmission in atmospheric channels.