Estimation of the capacity of provincial forest carbon sinks in Chinese mainland using the CO2FIX model provides data support for the effective management of provincial regions. According to China’s Sixth National Forest Inventory, we estimate the capacities of original and new afforestation carbon sinks under the assumption of using the country’s non-forest land for afforestation and reforestation to achieve a new forested area of 57323200 ha. The carbon absorption capacity of China’s forest ecosystems estimated from 2005 to 2050 reaches 8.4 GtC. The absorption capacities of original forest and new afforestation respectively are 4.9 and 3.5 GtC. The annual capacity of all forest carbon sinks has a roughly decreasing trend. Inner Mongolia Autonomous Region, Yunnan, Sichuan and Heilongjiang provinces make major contributions to the carbon sink capacity. Estimation of the capacity of provincial forest carbon sinks in Chinese mainland using the CO2FIX model provides data support for the effective management of provincial regions. According to China’s Sixth National Forest Inventory, we estimate the capacities of original and new afforestation carbon sinks under the assumption of using the country’s non-forest land for afforestation and reforestation to achieve a new forested area of ​​57323200 ha. The carbon absorption capacity of China’s forest ecosystems estimated from 2005 to 2050 reached 8.4 GtC. The absorption capacities of original forest and new afforestation respectively 4.9 and 3.5 GtC. The annual capacity of all forest carbon sinks has a roughly decreasing trend. Inner Mongolia Autonomous Region, Yunnan, Sichuan and Heilongjiang provinces make major contributions to the carbon sink capacity.