Reservoir inflow forecasting with high reliability plays an important role in the operation and management of the reservoir for power generation, irrigation, flood prevention as well as ensuring the safety of the dam. However, the level of forecast accuracy is limited, since its performance depends on rainfall forecasting and hydrological model. In order to increase the efficiency of forecasting, this study introduces the inflow forecasting method that integrates the real-time updating techniques with continuous optimization method of MIKE NAM model to specify the appropriate parameter set for forecasting time. The proposed forecasting method was tested for the Ho Ho reservoir, the area facing the scarcity of historical data for model calibration and verification. The analysis of the forecasting results for Ho Ho reservoir using transferred parameters from the stable calibrated parameter values at Hoa Duyet station (downstream of Ho Ho reservoir) and the results obtained using the adapted parameters by the proposed method shows that the adapted parameter values provides a more reliable forecast, which will better serve the decision making.