The effects of stand age and competitive types of trees (dominant and suppressed trees) on biomass allocation ratio and allometric relationship of each component were analyzed using ANOVA and multiple comparison based on a chronosequence approach for 7-, 17-, 30-, and 40-year-old Larix kaempfer plantations. We developed the high-precision allometric biomass equation as the basis of estimating biomass and carbon storage of Larix kaempfer. The results showed that:(1) the stand age had significant effects on the allometric relationship of biomass allocation. With the stand age increased, the proportion of stem biomass increased, but the proportion of branch and leaf biomass decreased, while the root biomass gradually became stable. The biomass model of stem, branch and leaf was significantly improved by adding the stand age variable. Age effect was the most significant in young stand stage, so biomass model should be constructed separately. (2) The tree competition types had also some effects on biomass allocation, but was less than that of the stand age. The dominant trees had more foliage biomass and less stem biomass. When the site conditions were similar, the competition effect could lead to the difference of biomass accumulation in the organs of dominant and suppressed trees, but the proportion and manner of biomass allocation to different organs were almost same. The competition within stand would not result in the biomass allocation strategy in terms of the theory of allometric relationship and the theory of environmental priority. So, when estimating the biomass of each component, the difference of tree grades in stand would not bring more deviation, The conventional method of estimating biomass by intermediate tree is feasible. (3) The root biomass of different competitive types of tree in pre-mature and mature stands was relatively stable. The percentage of root-stem ratio can be used to calculate root biomass.