To study the decay status of Korean pine (Pinus koraiensis) standing trees in natural forests,we selected and tested twenty six 20m×20m quadrats with different site conditions at fixed sample plots with a total area of 30ha, located at Liangshui National Nature Reserve, Lesser Xing'an Mountains. Six external decay indicators for a total of 129 Korean pine standing trees within quadrats were visually observed and recorded, namely swollen knots, swollen bole, canker, rotten branches, hollow and sporophore. The frequency, location and size of each indicator were statistically analyzed and used to estimate the decay rate of sample trees in the quadrats. Based on this, the relationships between decay rate and slope, slope aspect, slope position and elevation were analyzed. Then the relationship between decay rate and diameter at breast height (DBH) was emphatically discussed. The results showed that the external decay indicators of standing trees could be used to determine the decay status of forest trees, which also provide a potential method to rapidly estimate the decay degree of standing trees. Tree decay rate was influenced by environmental site factors. It correlated with slope negatively at a significance level P<0.05 and had a very significant correlation with slope aspect (P<0.01). However, it was not significantly correlated with both slope position and elevation (P>0.05). According to the regression analysis, the decay rate of sample trees presented a highly significantly positive linear correlation with DBH (P<0.01), which increased as DBH increased. When the DBH was larger than 60cm (the estimated tree age was 230 years), the decay rate was as high as 70%.