Northwest China belongs to the arid region, and there are many inland rivers in the region. There were frequent conversion relationships between surface water and groundwater in inland river basin. In order to explore the characteristics of transformation between surface water and groundwater, so as to provide theoretical basis for effective development and utilization of water resources and prediction of groundwater environment changes, the lower reaches of Tarim River was taken as the study area, and the river water and groundwater in different sections were sampled. What's more, conversion ratio of surface water to groundwater has been studied using isotope technology. Results show that: in the lower reaches of the Tarim River, conversion of river water to groundwater was remarkable, and the conversion ratio at Alagan section was the highest. In general, recharge of river water to groundwater presented a decrease trend along the river from upstream to downstream. Based on principle of mass balance and according to quantitative analysis, the average conversion ratio of river water to groundwater was 43.36%. This provides the foundation for evaluating ecological water conveyance efficiency correctly and is also helpful for rational utilization of water resources.