Based on the data of meteoro logy , hydrology, glacier change and human activities, the reasons of fluctuation in lake level in the Issyk-Kul, Kyrgyzstan, during the past 100 years were analyzed by means of water balance. It is found that precipitation and runof f are the determinants of the lake level change. Among the elements in water balance, precipitation and runoff are the most correlative with the change in lake level, with the correlation coefficient of 0. 76 and 0. 62, respectively. The irrigation water is the secondary one. Air temperature is increasing significantly in alpine area in the Issyk-Kul basin, precipitat ion and runoff feeding into the Issyk-Kul also increasing obviously. Therefore, the lake shrinking stopped in 1998, and then began to rise. At the end of 2008, Issyk-Kul lake level had increased by 0. 59 m. According to estimation, which prospects it will be warm in the future for all the SRES scenarios, the Issyk-Kul lake level will raise because the proportions of precipitation and meltwater increase continually . Therefore, the Issyk-Kul lake level has now been restored under natural fluctuation. It is no needed to transfer water to the Issyk-Kul from other basin. However, one should keep an eye on the water pollution that may be caused by the irrigation return flow .