To calculate the quantity and evaluate main functions of water retention services, a new method based on the WEP-L model that calculated the subtraction of precipitation and the surface runoff and canopy interception in the rainfall process as the water retention quantity was proposed, and the main functions of water retention were assessed such as decreasing floods (surface runoff), increasing basic flow (subsurface runoff), and the maintenance of vegetation ecosystems water uses (evapotranspiration). To verify the reasonableness of the new method, the calculation results using the new method and the InVEST method were compared, taking the upstream area of the Xianyang station in the Wei River Basin as an example. Since there are differences between the two methods in terms of assessment contents and application models, to ensure the comparability of calculation results, the results of the WEP-L model method I and the InVEST model method based on the same assessment contents were compared, and then the results of the WEP-L model method I and the WEP-L model method II based on the different assessment contents were compared. The conclusions are as follows: The calculation results of the WEP-L model method I and the InVEST model method were close, and the annual average quantities of the water retention service in the study area from 2000 to 2018 were 12.43 mm and 12.08 mm, respectively. The spatial distribution characteristics of the results using the two methods were basically similar, and the slight difference was influenced by the fact that parameters of the InVEST model have not been processed by the localization process; The results of the WEP-L model method I and the WEP-L model method II differed significantly, and the annual average quantities of the water retention service were 12.43 mm and 432.57 mm, respectively. The difference in the spatial distribution characteristics of the results of the two methods was distributed in the north, east, and northeast of the study area, which were mainly related to whether or not evapotranspiration was taken into the calculation process. Moreover, the multi-year trends of main functions (decreasing floods, increasing basic flows, and maintaining water use for the vegetation ecosystem) of water retention using the WEP-L model method II showed an obvious increase from 2006 to 2010, a significant decrease after 2012, and an increase, respectively. After 2012, there may be a trade-off between the functions of the increasing basic flows and maintaining the water use for the vegetation ecosystem According to the analysis of the reasons for the differences in the calculation results of different methods, it was indicated that the WEP-L model method II was more reasonable and could be used to evaluate the changes of main functions of water retention services, and the results could provide more basis for the strategies of water resources and ecological protection in Wei River water conservation area.