Based on the precipitable water vapor (PWV) data observed by Global Positioning System (GPS), the radiosonde data and daily precipitation data and employing several statistical methods, characteristics of summer PWV over the Tianshan Mountains and their possible reasons were analyzed. For the distribution of summer PWV average, remarkable differences exist among stations over the Tianshan Mountains, and there is a significant negative relationship between PWV and station altitude. The PWV at lower altitudes exhibits more variabilities and divergences than that at higher altitudes. The extreme and median of PWV in rainy days exceed those in rainless days. The daily and monthly variations of summer PWV are considerable, and most stations present maximum in July and minimum in August. As to diurnal variation, most stations have maximum at about 10 o’clock while some individual stations exhibit opposite features, and discrepancies still exit between rainy days and rainless days. There is no direct correlation between precipitation and PWV. However, precipitation and water cycle index are positively related to station altitude, which maybe be attributed to more local convective precipitation processes over higher mountains in summer and more water cycle frequencies.