Calibration and validation of any model before using for a specific study is essential. In this regard, the aim of this study was the calibration and validation of the FAO-AquaCrop model to predict the cassava yield in the Dong Xuan district belonging to Phu Yen Province of Vietnam. Based on the appraised results through calibration and validation processes whereby simulated biomass and crop yield under irrigation water came from rainfall during period 2000–18 could be concluded that the FAO-AquaCrop model, with the index of agreement (d) and correlation coefficient (R2) up to 0.85 and RMSE lower than 0.30 for simulating the biomass and cassava yields had confirmed the appropriate accuracy of the model performance. The results confirmed that the FAO-AquaCrop model was suitable for simulating the biomass and cassava yields in the Dong Xuan cassava cultivation area.