Abstract: The quantity of recharge of groundwater lateral flow from mountainous areas to plain areas is a long-term controversial and unresolved problem, which is always estimated to be too small or too large in arid inland basins in northwest China. Based on previous studies, this paper uses the technical methods of geological and hydrogeological investigation, geophysical exploration, drilling, pumping test, groundwater dynamic observation, hydrochemical test, basin groundwater level measurement and comprehensive study to identify the geological structure contact relationship, stratigraphic lithology contact relationship at the southern mountain-basin transition zone of the middle reaches of the Heihe River Basin, and the Cretaceous weathering crust aquifer structure and hydrogeological parameters at the Liyuan estuary. Through the analysis of the surface water and groundwater transformation relationships of different watershed levels in the mountainous areas, the recharge zones of groundwater lateral flow from the mountainous areas to the plain areas are divided into the large and medium-sized river valley recharge sections and shallow mountain recharge sections composed of small and micro rivers and gully groups. The exploration data of the river valley recharge sections are abundant, and the Darcy section flow method is used to calculate the recharge quantity. In view of the fact that the recharge section of the shallow mountain belt extremely lacks the exploration data, the runoff of the Liyuan estuary section is used as a reference. The estimation method of recharge of the groundwater lateral flow from the mountainous areas in the shallow mountain belt to the plain area is constructed with three variables of lithology, catchment area and precipitation. The results show that the quantity of recharge of groundwater lateral flow from the southern mountainous area to the plain area in the shallow mountain zone of the middle reaches of the Heihe River Basin is 0.40 × 10⁸ m³/a, and that of bedrock in the valley sections is 0.07 × 10⁸ m³/a. The quantity of recharge of groundwater of the Quaternary sediments in the valley sections is 0.30 × 10⁸ m³/a. The sum of the three recharges is 0.77 × 10⁸ m³/a, accounting for 3.0 % of the basin groundwater resources. This method provides an example for the estimation of the recharge of groundwater lateral flow from the mountainous areas to the plain areas in the northwest inland arid basins.