Abstract: Based on individual tree biomass of permanent sample plots from 2009 to 2014, 2014 to 2016 and 2016 to 2017 in Tiantai county, Zhejiang province, parameters such as average annual growth rate of biomass of reserved trees, average annual reserved rate of biomass, average annual number of ingrowth and their growth rate of biomass were calculated combining with ecological forest and commercial one, forest types and age groups. The biomass of each tree species group at subcompartment in 2017 was calculated for cardinal number of annual net productivity calculation in 2018. The productivity of reserved tree, potential one of felled or mortality and ingrowth of different species at subcompartments was calculated combined with responding parameters, and then the annual net productivity of arbor layer of forest in the county could be calculated. The results showed that in 2018, the net productivity of arbor layer of forest in Tiantai was 5.895×105 t of which 5.404×105 t of reserved tree, 0.125×105 t of potential, 0.366×105 tons of ingrowth, and annual net productivity per unit area was 7.00 t/ha. The net productivity of ecological forest was 3.523×105t, and that of commercial one was 2.372×105 t. The annual net productivity of coniferous-broad-leaved mixed forest was the highest, 1.849×105 t, and that per unit area of hard broad-leaved forest was the largest, 0.833×105 t. The annual net productivity of middle-aged forest was the highest, 2.782×105 t, and that per unit area of young forest was the largest, 0.833×105 t.