Optical beam forming network has the characteristics of large bandwidth, low transmission loss, and anti-electromagnetic interference, compared with the traditional electrical beam forming network. In the case of multi-channel radio frequency signal input, the law of insertion loss of optical beam forming networks based on the dispersion fiber system is less studied. Assuming that the photon detector reaches the saturation threshold, the insertion losses of the single-channel microwave photon link and the dual-channel optical beam forming network are calculated. It is deduced that as the number of input radio frequency channels increases by multiple folds, the insertion loss of the optical beam forming network using a dispersion fiber also increases in an alike manner. A test system was set up, and verification was carried out. When the number of input radio frequency channels was 2, 4, 8, and 16, the corresponding insertion loss was -26.0 dB, -30.8 dB, -34.3 dB, and -46.0 dB, respectively.