Abstract: The distribution law of UHF electromagnetic field strength in rectangular tunnel was studied by use of geometrical optics model and experimental data, and conclusion was obtained: there are apparently different electromagnetic field distributions when transmitting antenna is located in different positions of a tunnel section. When the antenna is in the middle of the tunnel, the fast fading convergence speed is the fastest and received electromagnetic field strength is the maximum. The electromagnetic field strength exhibits a nearly cosine scattering distribution, while it distributes relatively even when the location of transmitting antenna is nearby the tunnel wall. The conclusion can be used to predict effective coverage range of electromagnetic field in tunnels, and is useful for design and detection of cellular personal mobile communication network in tunnels.