Spatial relationships are important factors affecting forest growth and community succession, and interspecific relationships are important components of it. The study of spatial relationships helps to understand the growth, functional roles, and successional processes of forest communities. The correlation of interspecific relationships is easily influenced by sample size and the number of samples taken. Existing studies often use either small sample sizes with a large number of samples or large sample sizes with a small number of samples to ensure accuracy. Conducting multi-scale studies with a large number of samples can be difficult for data analysis. This study avoids these limitations by using the moving window method and combining it with Pearson correlation coefficient to analyze the interactions and variations between different diameter classes of Pinus koraiensis and associated tree species at different scales in the Liangshui National Nature Reserve (10.4 hm² sampling plot) in Heilongjiang Province, and to conduct multi-scale comparisons. The study obtains the spatial relationship characteristics of Pinus koraiensis and main associated tree species at different scales and different diameter classes, explores the interaction mechanisms between Pinus koraiensis and associated tree species, helps to understand the ecological adaptability and dynamic changes of Pinus koraiensis in communities, and reveals the selection and adaptation mechanisms of dominant tree species Pinus koraiensis to habitats, providing references for the better protection of Pinus koraiensis forests. The results show that: (1) the mutual relationships between Pinus koraiensis and associated tree species are diverse and exhibit multiple coexisting relationships, demonstrating the complexity and stability of the broad-leaved Pinus koraiensis forest community structure. Large size-class Pinus koraiensis suppresses associated tree species of the same size class, showing a highly significantly negative correlation, while they provide shade for small size-class associated tree species, showing a highly significantly positive correlation. (2) The mutual relationships between Pinus koraiensis and major associated tree species show significant differences at different scales and size classes. The larger the scale, the smaller the rate of change in correlation coefficient between Pinus koraiensis and associated tree species. The larger size-class associated tree species have a larger rate of change in the correlation coefficient with Pinus koraiensis. (3) The correlation between Pinus koraiensis and associated tree species mainly manifests at large scale (≥60 m) and is weak at small scale (＜60 m), exhibiting scale characteristics. The combination of Pinus koraiensis and associated tree species at the same size class and different size classes show different spatial relationships. The interspecies relationships between Pinus koraiensis and associated tree species of the same size class mostly show negative correlation, while the interspecies relationships between Pinus koraiensis and associated tree species of different size classes show positive correlation. (4) Based on interspecies relationships, the minimum plot size in the Liangshui National Nature Reserve is determined to be 60 m×60 m. This study focuses on the interspecies relationships between different size classes of Pinus koraiensis and associated tree species, explaining the spatial relationships of dominant species Pinus koraiensis and associated tree species in the forest, and providing reference opinions for future forest management.