Abstract: The forest is the main part of water conservation ecosystem in Qilian Mountains，Gansu Province，China，which plays a huge ecological service function. Changes of forest resources have significant impact on the structure and function of the forest ecosystem. Currently，quantitative methods are mainly used to study the changes of forest resources. It is vital to investigate the changes of forest resources for developing the forest protection policies and regulations，protecting effectively and using reasonably forest resources in Qilian Mountains. From 2010 to 2013，the changes of forest distribution and area in north slope of Qilian Mountains were comprehensively analyzed by applying 3S technology and collecting the previous research data. The results showed that， during the geological period from Silurian （440 Ma） to Holocene （0.01 Ma），along with the rapid uplift of Qinghai-Tibet Plateau，the landform in Qilian Mountains experienced a dramatic change from ocean to mountains，then higher mountain，and the forest gradually appeared and developed to the whole mountain system but degenerated to the east area to Jiuquan City later. From the late Neolithic Age to the late Warring States Period （2 986-221 BC），the area of forest was approximate 600×10⁴ hm². In 1958，the retained area of woodland and shrubland was 12.4×10⁴ hm² and 20.3×10⁴ hm² respectively. In 1980，the area of woodland decreased by 4.8% compared with it in 1958. In 2000， the area of woodland and shrubland increased 6.0×10⁴ hm² and 12.1×10⁴ hm² respectively compared with it in 1980. In 2013， the area of woodland and shrubland increased 9 733.4 hm2 and 36. 4×10⁴ hm² compared with it in 2000. During the historical period from Han Dynasty to the early stage of New China， forest in north slope of Qilian Mountains suffered severe damage after four large-scale deforestation. From 1960s to late 1980s，the area of forest was decreasing constantly owing to 3 massive reclamation and deforestation. Since the 1990s，especially scince the 21st century，by strengthening protection，forest area have been gradually increasing. Recently，because of the intensified human disturbance，some local forest area is still in decline，and the quality of forest is decreasing. Analyzing the reasons for the changes of forest resources，the paper put forward that during geological period，changes of forest resources were completely under the restriction of the natural environment， especially climate change，and during the historical period， they were significantly influenced by human activities. In view of the existing problems，the paper suggest 6 measures to solve them，which are defining the ownership of forest，building use control system of forest resources，implementing the evaluation system of goals to increase the forest resources，improving the mechanism for ecological compensation，relocating farmers and herdsmen in forest，and implementing key projects of ecological protection and restoration.

Key words: forest, distribution, area, change, north slope of Qilian Mountains