By means of multi-source and multi-temporal digital remote sensing images as well as hydrometeorological datas around Hulun Lake, high-resolution image reconstruction methods based on convolutional neural networks were used to study the change of the area and storage capacity of Hulun Lake from 1999 to 2019. The results show that the area of Hulun Lake has decreased year by year from 2003 to 2012, while the water volume of the lake has decreased year by year. The multi-year average runoff of Wuerxun River and Kerulen River supplying Hulun Lake from 2003 to 2012 was 130 million m³ and 141 million m³ respectively, which accounts for only 21% and 24% of the multi-year average runoff before 1991. From 2003 to 2012, the average annual evaporation from water surface of Hulun Lake was 1.75 billion m³ and the average annual precipitation of the lake surface was 0.325 billion m³. The average annual amount of groundwater replenishing Hulun Lake was 0.53 billion m³. The main source is Cenozoic basalt groundwater and the volcanic basalt groundwater at the source of the Halaha River is leaked through lava pipes. Based on this, it is inferred that the groundwater replenishing Hulun Lake comes from the allogenic water across the basin.