Water resource plays an important role in social and economical activity in China, especially in arid or serai-arid northern regions. In the ungauged Hulun Lake Basin, water evaporation was estimated by Penman -Monteith equation by using daily meteorological records at Manzhouli Station, and calibrated with monthly pan evaporation at Alatan'emole Station. A two-parameters monthly water balance model was developed in the study to reconstruct the hydrological series. 42-years (1961-2002) hydrological series were reconstructed for the Hulun Lake by using the model. Simulated water level is good agreement with the observed annual mean data. The estimation of water resources is reliable, which can provide solid base for wise management of water resources in Hulun Lake.