Vegetation productivity is closely associated with the productive and daily activities of human beings. Vegetation provides them with food, raw materials and fuels, and also has such functions as wind prevention and sand fixation and air purification. Moreover, they are key factors in the regulation of carbon source and carbon sequestration of the ecosystem. The absorption of carbon dioxide and release of oxygen by the green plants through photosynthesis helps to strike a balance of CO_2 and O_2 in the atmosphere. In particular, with developments in socioeconomic sectors, a sharp rise in the CO_2 concentration in the atmosphere as well as abnormal conditions in the global climate, vegetation's ability to fix carbon and release oxygen and the value thereof appear to be of particular importance. Based on the remote sensing data and measured data at meteorological stations from 1980 to 2010, we have evaluated the net primary productivity of the vegetation in Northern Shaanxi using the CASA model (a refined version from the light energy utilization model), calculated the value of carbon fixation and oxygen release of the vegetation and analyzed the characteristics of its changes. The findings suggest the following: (1) There is a major difference in different land- use patterns. In a descending order, the NNP averages are respectively forest land, grass land, arable land, construction land and unused land. From 1980 to 2010, the variation in the total value of NPP has been featured by the trend of "increase-decrease-increase", with the amount of NPP rising by 35.60% over the past 30 years; (2) In a descending order, the amount of carbon fixation and oxygen release for different land- use patterns are successively grass land, forest land, arable land, unused land and construction land; and (3) In 1980, 1990, 2000 and 2010, the total value of carbon fixation and oxygen release are respectively 267.208*10~8 yuan, 276.725*10~8 yuan, 260.796*10~8 yuan and 362.318*10~8 yuan. During the period of 1980-2010, the total value of carbon fixation and oxygen release have been increased by 95.110*10~8 yuan. In terms of spatial distribution, Fugu and Shenmu counties in the southern and northern parts of Northern Shaanxi have seen marked fluctuations in the total value of carbon fixation and oxygen release, while the northwestern part of Northern Shaanxi has seen mild interannual variation in the total value of carbon fixation and oxygen release; in terms of land-use patterns, the standard variation has shown a trend of "forest land, grass land, arable land, unused land and construction land" in a descending order. (4) Over the past 30 years, the total value of carbon fixation and oxygen release has shown a general trend of continuous increase. In terms of land- use patterns, the average rate of change in the total value of carbon fixation and oxygen release has respectively been unused land, construction land, arable land and grass land in a descending order. (5) On the whole, the value of carbon fixation and oxygen release in an ecosystem is positively correlated with the precipitation and negatively correlated with the monthly average temperature. Changes in the value of carbon fixation and oxygen release typically respond intensely to those in precipitation. The temperature has a great impact on the value of carbon fixation and oxygen release by coupling with other climatic systems. The difference in land coverage has been a major factor contributing to the marked difference between the northern and southern parts of Northern Shaanxi in the value of carbon fixation and oxygen release in terms of spacial distribution. As the southern part is located in the semi-humid region with a high ratio of vegetation cover and is dominated by broadleaved deciduous forest, the amount of CO_2 fixation and O_2 release is relatively large.