Abstract: To search after the adaptability on site and suitability on density of Larix Principis-rupprechtii Mayr.stand for water resource conservation,the growth process and diameter distribution characteristic of Larix Principis-rupprechtii Plantation in 10 years old were studied using stem analysis method of sample tree,which had two density structures(one was 2 600 per hm2,the other was 3 500 per hm2) in water conservation forest region of Wutai Mountain in Shanxi Province.The results showed that: 1) The tree height growth of two kind of forests were the same basically,and it was almost not affected by density.But the growth process of diameter and timber volume had great distinction.The growth status and its density structure of low density forests were superior to the high density forests.2) The S_kewness(S_k) of diameter distribution had great distinction.The S_k(was 0.01) of low density forests approached to normal distribution,the density structure was reasonable,while the S_k(was 0.45) of high density forest was partial to normal distribution,the density structure was on the high side.The kurtosis(K) of two forests(one was-0.64,the other was-0.74) had little distinction and the density factor had little function to forest polarization.3) The growth of diameter at breast height,timber volume and trunk stock of low density forests increased year after year without effect of density.But the growth of high density forests had declined from since 6 years old,which was restricted by high density.4) The reasonable density of Larix Principis-rupprechtii Plantation about 10 years old was 2 600 per hm2,if the utilization of double cutting was not considered,it is also the reasonable planting density.