Community composition and structure are the foundation of ecological system function and process research. Through a comprehensive understanding of community composition and structure, important information was obtained for further study of species diversity, species coexistence laws, and formation mechanisms. To study species composition and community structure of a coniferous and broadleaved mixed forest, a 21.84 hm² (420 m × 520 m) permanent forest plot was established at the Jiaohe Forestry Experimental in Jilin Province in July 2010. We measured parameters for each woody plant with diameter at breast height (DBH) ≥ 1.0 cm. These parameters included the number of individuals and species, DBH, tree height, height under the first living branch, crown width, tree growth condition, and others. Structure characteristics of the community in this plot were analyzed based on the species composition, species-area relationships, floristic features, diameter class distribution structure, and spatial distribution patterns. A total of 25,908 living individuals were documented, belonging to 45 species, 27 genera, and 18 families, with a stem density of 1,186 individual/hm². At the genus level, the North Temperate distribution type was the main vegetation type, which has 37 genera, accounting for 82.22% of all genera. There were 18 and 14 species of rare (≤ 1 individual/hm²) and accidental species (1-10 individual/hm²), respectively, in the plot. The Margalef diversity index R, Shannon-Wiener index H', Simpson diversity index D, and Pielou evenness index Jsw were 4.31, 2.48, 0.89, and 0.65, according to the equations (2)-(5), respectively. The diameter class distribution of all individuals (2cm class) followed a partial reverse "J" shape, which indicated that the natural regeneration understory was good and the forest community exhibited a steady growth trend in this plot. The diameter class distribution of nine species with importance values >3 exhibited some differences: J. mandshurica and F. mandschurica had an approximate normal distribution; U. davidiana var. japonica, U. macrocarpa, A. mono, A. mandshuricum, and U. laciniata exhibited an inverted "J" shape or partial inverted "J" shape, which suggested regeneration was good; P. koraiensis exhibited an irregular pattern because of severe human disturbance and poor natural regeneration. The Paired-correlation g(r) function results indicated that the nine primary tree species had aggregated distributions, and the aggregation intensity decreased with increasing distance. The aggregation intensity decreased dramatically when distance (r) was <20 m, but the trend became steady when distance was > 20 m.