In order to analyze the friction law of the rectangle channel, this experimental research is carried out in an artificial rough-wall channel under three different roughness coefficients The experimental results indicate that, under an uniform flow, the roughness coefficient is not constant in the same roughness channel when the hydraulic condition changes. It increases logarithmically when the width-depth ratio of the channel increases, and decreases logarithmically when the flow increases, or decreases followed a power function when the Reynolds and Froude number increases, respectively. Meanwhile, the results show that different wall roughness has an obvious effect on the roughness coefficient, and then affects the hydraulic characteristics. Namely, water depth increases when the wall roughness increases, while mean velocity of flow and the Reynolds and Froude decrease under the same discharge condition. At last, the relation formula between roughness coefficient and equivalent roughness is given according to the experimental data.