There are a large number of lakes on the Ordos Plateau. Lake evaporation, as the main discharge mode of surface water, is one of the key equilibrium items for the accurate evaluation of water resources. Studying lake evaporation has important practical significance and scientific value for the understanding about regional water cycle, the evaluation of water resources, and the ecological and environmental protection of lakes. However, the measured data of lake evaporation in this region are limited, and most of them are calculated with conversion method and formula method. In this study, an in-situ test was carried out on Mukainao, a typical lake on the Ordos Plateau. An evaporator was installed in the middle of the lake to measure the water surface evaporation and analyze the correlation between evaporation and meteorological factors, and the in-situ lake evaporation was compared with the pan evaporation of lakeside land surface and the theoretical value of Penman Formula. It is shown by the study that lake evaporation is greatly affected by humidity and air temperature; the conversion coefficient between the water surface evaporation of lake and the small pan evaporation on lakeside land surface is about 0.85, larger than previous studies; the lake evaporation is little affected by the salinity of lake water and can be ignored; the correlation analysis result is relatively high compared with the calculated results of Penman Formula, so Penman Formula can be used to estimate lake evaporation in the absence of measured data.