Both liquid water path (LWP) and precipitable water vapor (PWV) are two important variables describing water source in the air, which are key elements in the energy budget and the workings of the hydrology. Currently, direct and continuous observations of LWP and PWV are few, especially for the Loess Plateau arid and semi-arid areas. The measurement of microwave radiometer at the Semi-Arid Climate and Environment Observatory of Lanzhou University (SACOL) is used to analyze the diurnal and annual variations of cloud liquid water path (LWP) in the semi-arid area. Firstly, Liljegren et al. retrieval method (Liljegren et al., 2001) is introduced and improved, and then used to calculate the retrieval coefficients suitable for the Loess Plateau, and LWP and PWV are computed using the modified retrieval method in the past two years. Compared with the artificial neural networks retrieval results of TP/WVP-3000, the improved Liljegren et al. retrieval results in this paper have smaller errors from the reality. The results show that 95% of LWP is below 150 g/m2, and 95% of PWV is below 3 cm at the SACOL. Because of the influence of Asian monsoon on precipitation, the daily average of LWP is the smallest in winter, the largest in autumn. The diurnal variation shows two peaks, which is due to local climate features, such as temperature inversion and thermal convection. Furthermore, the authors compare the annual variations of LWP derived from Terra and Aqua satellite with that from microwave radiometer, respectively. The results indicate that the satellite-derived results are close to the ground-based retrievals. Therefore, using LWP retrieved by satellites data to analyze the water source in the air over Northwest China is a relatively reliable method.