Evapotranspiration (ET) is the major cause of wetland water loss. The Penman–Monteith model is the most suitable ET model for wetlands. However, its accuracy depends on canopy resistance. Here, we studied the Phragmites australis community in the Liaohe Delta, northeastern China. We used flux and environmental data from the Panjin Wetland Ecosystem Research Station, and physiological and ecological parameters. Canopy resistance was calculated by the Penman–Monteith model, and canopy resistance and its influencing factors were analyzed. We created a canopy resistance model, named the Phragmites australis wetland (PW) model, using leaf stomatal resistance, leaf area index (LAI) and environmental factors. The PW model differed from the traditional Jarvis model in that the effective LAI was added, and the stomatal resistance was changed from a fixed to dynamic value and the environmental factors only contained two items: solar radiation and water vapor pressure difference. The PW model allowed the conversion from leaf scale to canopy scale. A comparison of the PW model with the Jarvis model parameters showed that accuracy improved significantly: R2 values increased from 0.56 to 0.74. The model can provide parameters for P. australis ET models and provide a new method for accurate estimation of wetland ET.