<p>Seedlings and saplings are critical for forest dynamics and regeneration, however, their growth rules and biomass allocation strategies under forests are still largely unknown. Here, we destructively harvested a total of 74 <i>Pinus densata</i> seedlings and saplings aged from 2 to 24&#xa0;years in western Sichuan from both open and understory habitats. We measured age, basal diameter, height, and biomass of total-tree, foliage, branch, stem, coarse root, and fine root of each sample tree, to investigate the early growth process and biomass allocation patterns, as well as their differences between habitats, and develop biomass estimation models for total-tree and its components. We found that: (1) basal diameter, height, and biomass of seedlings and saplings increased with age following power laws, which is the same as adults, but their growth rates differed in different growth stages; (2) seedlings and saplings in open habitats had larger basal diameters but similar heights compared to understory counterparts, with no significant differences in growth rate; (3) seedlings and saplings in open habitats had greater total, aboveground, root, foliage, and coarse root biomass than those in understory habitats, although biomass accumulation rates were comparable between the two habitats; (4) at seedling and early sapling stages, pines in open habitats allocated a higher biomass fraction to roots, whereas understory pines favored stem growth; (5) The optimal predictor for estimating total-tree biomass is basal diameter (<i>D</i>) in practice, with the model as ln(<i>W</i>) = −3.79822 + 2.49630 ln(<i>D</i>). These findings show that the growth and biomass allocation of <i>P. densata</i> seedlings and saplings, deviating from the observed patterns in adult trees, are affected by habitat types, and can be used to guide forest management and carbon assessment.</p>