In this study, Beichangshan Island in Changdao County of Shandong Province was used as the research area. On the basis of field investigation, the methods of mathematic model and 3S technology were adopted. The plantation biomass was classified as total biomass (TB), existent biomass (EB), and lost biomass (LB), which represents the biomasses of all trees, living trees and dead trees, respectively. The ratio of existent biomass to lost biomass (EB/LB) was used to describe the relationship between different components of biomass and to reflect the plantation health condition from a new perspective. Then, the spatial distribution patterns and the impacting factors of plantation biomass were discussed. The results revealed that TB of plantation on Beichangshan Island summed to 58.8 kt, with the average value of 145.8t/hm², which was higher than that of the whole country and Shandong Province. EB summed to 37.9 kt with the average value of 93.9t/hm², and LB summed to 20.9 kt with the average value of 51.9t/hm², indicating that the health condition of plantation has been an important factor impacting the plantation biomass. With the increase in stand density, both TB and EB firstly increased and then decreased, while LB kept an increasing trend, whereas EB/LB showed the opposite trend. The diameter at breast height (DBH) of 10~20cm accounted for the most biomass, and with the increase in DBH, EB/LB of Pinus thunbergii tended to decrease, while EB/LB of Robinia pseudoacacia showed the opposite trend. The normalized difference vegetation index (NDVI) was significantly positively correlated with TB, EB and EB/LB, but negatively correlated with LB. EB/LB was significantly positively correlated with slope aspect, soil moisture content, total phosphorus and total nitrogen but negatively correlated with altitude, slope, soil salinity and total potassium, and had no significant correlations with slope position, soil pH and total carbon.