Species composition and populations' diameter class structure were investigated in a 1.1 hm² permanent plot in the coniferous and deciduous broadleaved mixed forest in Xiaoxing'an Mountain. Using O-ring functions in the spatial point pattern analysis. the populations' spatial distribution patterns and spatial association are analyzed. The results showed that there were 15 arbor species in the plot. Picea koraiensis, Betula costata, Ulmus davidiana, and Acer mono ranked as the top 4 important species in our plot. Picea koraiensis and B. costata were the major dominant species with importance values of 15.81% and 11.10%, respectively. This forest was also founf to be in good regeneration condition. The size class structure of B. costata tended to be normally distributed, while that of the other three populations showed an inverted "J" shape. The results from the O-ring function showed that the spatial distribution pattern of the four main populations was similar, with distributions was aggregated at the 0-5 m scale. At increasing scales, distributions shifted from aggregated to random. Picea koraiensis had a significant positive or no correlation with the other three main populations at coarse scales, but only had a significant positive correlation with B. costata at fine scales. Acer mono had a non-significant negative correlation with the other three populations at fine scales. As the scale increased, no correlation was found.