Assessment of forest naturalness plays an important role in biodiversity conservation and forest management. In the present study, we selected thirty-five plots from the main forest types, and investigated 30 indicators (three topographic indicators and 27 community indicators) in the Baotianman National Nature Reserve. Community indicators (27) were analyzed using the PCA method to determine forest naturalness index values (N). N values were clustered using the Ward cluster method, and naturalness classes were classified according to those cluster groups. The results showed that: the degrees of forest naturalness were divided into five grades: highest naturalness plots: with N values ranging from 2.18 to 1.13, an average stand age of 94a, and total plots accounting for 20.0%; higher naturalness plots: with N values ranging from 1.01 to 0.34, an average stand age of 80a, and total plots accounting for 34.3%; medium naturalness plots: with N values ranging from 0.01 to -0.47, an average stand age of 47a, and total plots accounting for 17.1%; lower naturalness plots: with N values ranging from -0.92 to -1.60, an average stand age of 26a, and total plots accounting for 14.3%; and lowest naturalness plots: with N values ranging from -1.98 to -2.54, an average stand age of 21a, and total plots accounting for of 14.3%. Among the first three principal components, soil bulk density, soil organic matter, total nitrogen, nitrate nitrogen, stand age, mean diameter at breast height (DBH) of the dominant trees, and diversity indices of the arbor, shrub, and herb layers were all greater than the other indicators, and were key factors that influenced forest naturalness. The N value was significantly negatively linear when fitted to soil bulk density (R² = 0.763, P < 0.0001), and significantly positively linear when fitted to stand age, and Shannon-Wiener indices of shrub and arbor layers and altitude, respectively (P < 0.001). Logarithmic curve fitting reached significant levels between N value and soil organic matter, and total nitrogen, respectively (R² > 0.794,R² = 0.815,P < 0.0001). Greater forest naturalness was observed at sites at an altitude of 1118-1863 m. At an altitude of <970 m, forest naturalness sharply decreased. Based on the different classes of forest naturalness, some corresponding measures for discussion and management were proposed.