Flow friction is the key to studying water movement and has been one of the most important research topics in hydraulics and river dynamics. The roughness coefficient in the Manning formula represents friction applied to the flow by channel and changes with the river section characteristics, water level, and flow velocity. However, the Manning formula tends to simulate the friction with little variability, which contributes to large errors in the simulation of water level and flow velocity. To solve this problem, we proposed an improved friction formula based on the relationships between roughness coefficient and energy gradient and developed a differential model of one-dimensional flow with the proposed friction formula. The developed model was tested against both the experimental flood data and observed flow data in Qiantang River, China. The results indicated that the proposed friction formula provides a better simulation of target friction than the original Manning friction formula. The parameters in the proposed friction formula are less sensitive to the river section characteristics. Our results also showed that the developed differential model using the proposed friction formula can simulate the water level and flow velocity well in both the calibration and validation period and can improve the simulation of water level in tidal reach.