Through recent extensive studies on air quality, researchers have paid more attention to changes in oxygen content in the atmosphere. Studies show atmospheric oxygen in the lower atmosphere has a slow diminishing trend. The forest ecosystem is an important source of carbon sequestration and oxygen release, so, oxygen content is directly impacted by the change of forest area. Therefore, it is essential to examine the deforestation process and evaluate its impact on the changes of oxygen content in the atmosphere. Heilongjiang Province is the area in China primarily focused on forest resources and coverage, however, during the past century approximately 37.163% of the forest area has been converted to other land uses. We selected Heilongjiang Province as the research area, as it provided greater significance to study the variation of oxygen content in the atmosphere due to the estimated change of oxygen release caused by deforestation. According to historical references and remote sensing image data, we rebuilt the forest spatial distribution of Heilongjiang Province in 1900, 1949, 1986 and 2009. Using the C-FIX Model and carbon-oxygen balance method, this research simulated and analyzed the quantity and changes of oxygen released by the forest ecosystemof Heilongjiang Province.Analyses of the results suggest four major conclusions. First, the forest area of Heilongjiang has decreased by 106667.570 km², or approximately 23.568% during the last century (1900-2009). Except for Pinussylvestris var. mongolica forest and Coniferous and broad leaved mixed forest, the geographic areas of all other forest types have decreased, within which the areas of Piceaasperata and Abiesnephrolepis forest and Pinuskoraiensis forest have decreased by more than 80%.Spatially, significant forest loss can be found in the Songnen and Sanjiang Plains. Second, during the last century, the quantity of oxygen released has been reduced by 56.2156 million tons, or approximately one-third of that in 1900. The oxygen production in the Southwest was the highest in 1900, but became the area with the lowest production in 2009. Moreover, a significant decrease of forest oxygen production occurred in the Sanjiang Plain. Third, the quantity of oxygen released from almost all administrative regions of Heilongjiang presents a decreasing trend, with the exception of Yichun. Among these regions, the quantity of oxygen released has reduced more than 90% in Daqing City and Qiqihar City; over 50% in Jiamusi City, Suihua City, and Jixi City; over 25% in Shuangyashan City, Qitaihe City and Heihe City; less than 10% in Daxinganling Region and Mudanjiang City. Fourth, during the past century,the total amount of oxygen released by Picea asperataand Abies nephrolepis forest has reduced by 91.855%, 78.905% by Pinuskoraiensis forest, and by 25.043% and 23.936% by deciduous mixed forest and Larixgmelini forest, respectively. However, the total amounts of oxygen released by Coniferous and broad leaved mixed forest and Pinus sylvestris var. mongolica forest increased by 20.356% and 48.209%,respectively. Piceaasperataand Pinus koraiensis forest released the most amount of oxygen per unit area; Larixgmelini forest and Pinus sylvestris var. mongolica forest released less. Finally, the results of the control experiments suggest that forest oxygen production in Heilongjiang is only approximately half of the amount in 1900.Moreover, forest degradationdue to human activities is one of the major factors that leads to the decrease of the quantity of oxygen released.