Phragmites australis wetland in the Liaohe River Delta of Northeastern China was selected as a research object. Based on the dual crop coefficient of FAO-56 model and the observed data (including flux data, microclimate gradient data, water evaporation, growth parameters of P. australis, etc.) in Panjin Wetland Ecosystem Research Station, the crop coefficient (K_c), the basal crop coefficient (K_cb), and the water evaporation coefficient (K_w) were calculated. The diurnal dynamics of three coefficients and their influencing factors were analyzed. Dual crop coefficient model at hour scale was established using the biological factors and environmental factors. The results are as follows:(1) In the initial growth stage, the diurnal variation of K_c and K_cb showed multi-peak fluctuation with higher values in the morning and evening and lower values at noon; during the rapid development and stable stage, the fluctuation ranges of K_c and K_cb were smaller in the daytime and larger at night; at the end of growth season, K_c and K_cb fluctuated greatly at night and presented multi-peak curves in the daytime. K_w was smaller in the daytime and higher at night, with the daytime values in the initial stage higher than those of the other stages. (2) The correlation analysis showed that the influencing factors of K_c, K_cb, and K_w were air temperature, relative humidity, wind speed, plant height, and leaf area index. The dual coefficient model was reconstructed with the biological and environmental factors. Thus, the simulation of the evapotranspiration in P. australis wetland was improved with the determination coefficient R² reaching 0.894. (3) Using the improved dual crop coefficient model and FAO-56 model, the evaporation and transpiration of P. australis wetland in the Liaohe River Delta were simulated. This could solve the problem that transpiration is unobtainable from direct observation and improve the accuracy of evapotranspiration simulation. (4) We adjusted the constant value of P. australis crop coefficient recommended by FAO which was more suitable for the Liaohe River Delta wetland.