The ability to accurately age a forest has always been an important component of forest ecology research. Traditionally, the diameter of a tree at breast height (DBH) has been used to estimate tree age and subsequently identify the age structure of a forest. Unfortunately, the DBH index is not an accurate estimator of tree age for natural Korean pines (Pinus koraiensis) because of the complexity of spatial distribution and different developmental stages of this species. Previous studies have examined the relationship between DBH and age of natural Korean pines using a simple regression analysis. In our study, the wood data of 156 mature Korean pines in the Lesser Khingan Mountains of the Heilongjiang province was collected to analyze the relationship between DBH and age of Korean pine, as well as the influences of topographic and competition factors. In our wood data analysis, we preformed simple regression matching with an exponential model to better suit the biological significance and distributional characteristics of age for natural Korean pines. In addition, we adopted the widely used Hegyi simple competition index (CI) and improved Basal-Area-in-Larger Trees (WBAL) as competition indexes. We imported the competition indexes into the (DBH-age) regression equation and analyzed them by different slopes and slope positions using multiple regression analysis to understand how topography and competitive factors influence the relationship between DBH and age of natural Korean pines. Our results showed that the Pearson correlation coefficient of DBH and age was 0.562, and for our wood data collected and analyzed, the linear correlation between the DBH and age was not high. When we fit our wood data with exponential models the determination coefficient R² was only 0.507. However, when we imported the competition indexes for multiple regression analysis, the adjusted determination coefficient of the regression model increased to 0.542; when we imported the WBAL index, the fitting effect of DBH-age improved to 0.583. We conducted fitting analysis using different slopes and slope positions and found that natural Korean pines in the middle slope and southward slopes showed the best fitting effects between the DBH and age. However, after importing the competition factors, we significantly improved our precision in estimating age, especially with pines in the northward and westward slopes. For example, after WBAL was imported, the determination coefficient for DBH and age increased by 0.355 and 0.269, respectively. Our study found that altitude, slope, aspect, and competition factors could all influence the relationship between DBH and age of natural Korean pines. Among them the aspect and slope position had the strongest influence; however, the competition factors showed limited influence, depending on topography. However, the underlying mechanism of effects of topography and competitive factors on the relationship between DBH and age of Korean pine is complicated. This article has only carried on the preliminary exploration. More investigations should be conducted in the future to further improve fitness of the relationship between DBH and age of Korean pine.