Abstract: This paper reports the result of study on the relationship between afforestation density and biomass of Cunninghamia lanceolata (Lamb.) Hook young growth. The result shows that when at young growth stage, the biomasses of leaves, roots, branches and total in a given stand, increase with the increase of afforestation density until it reaches 6 567 trees/ha. Then, the total and stem biomass begin to decrease as the density increases or decreases, and the increment of leaves, roots and branches becomes smaller. High afforestation density raises the rate of biomass of small diameter classes, while low density decreases the total biomass. The leaf area index also increases with the increase of afforestation density when the density in no more than 6567 trees/ha. If density surpasses 6567 trees/ha., the increment becomes smaller. Too high or too low afforestation density results in the decrease of efficiency of net photosythesis of leaves.