Through the analysis of test data of physical model, The paper discussed the regularity of roughness changing with water depth and Froude number in rectangular channel. The result showed that when slope is constant,with the increase of Fr the roughness n value decreases gradually. In the slow flow channel,the roughness value of n changes quickly with Fr; in the jet stream channel, the roughness value of n changes more slowly with Fr. That the roughness n changes with water depth relates to flow pattern. With the increase of water depth h, the value of n decreases gradually; when Fr is larger than 1 and less than 1.51, the value of n decreases first with the increase of h and then increases with the increase of h at a certain number; when Fr is larger then 1.51,the value of n increases with the increase of h.