The assessment of suitability is the cornerstone for the development of ecotourism in nature reserves. This paper adopts the Delphi method to invite 30 experts to score and screen a series of indicators and then calculates the weight of each indicator through the hierarchical analysis method (AHP) to establish a comprehensive evaluation index system for the suitability of ecotourism development. The AHP method includes four constraints layers (tourism resources, socio-economic environment, ecological conditions, and tourism market), in addition to eleven element layers and thirty-eight indicators. It establishes overarching criteria for evaluating ecotourism suitability. Our research focuses on Dinghushan, Xilin Gol Grassland, Hongze Lake Wetland, and Jiuzhai valley, and the results are as follows: (1) Ecotourism suitability evaluation level is divided into five levels, level I (0 ≤ S < 30) ecotourism development suitability is the lowest, meaning an area is extremely unsuitable for ecotourism development. Level V (90 ≤ S < 100) has a very high ecotourism value, meaning an area is highly suitable for ecotourism development. (2) Jiuzhai valley scored the highest ecotourism suitability evaluation score of 87.63, and Xilingol Grassland scored the lowest score of 81.27. However, the composite scores of all the nature reserves were above 80, placing them at Suitability Level IV, and thereby indicating a high suitability for ecotourism development. (3) Divergences in ecotourism suitability emerge among various nature reserve types, with grassland and meadow reserves exhibiting lower suitability levels. Addressing this, a robust management and monitoring system is imperative, alongside intensified efforts in ecological restoration, vegetation protection, community engagement, education, awareness, and increased policy support and tourism capital investment. (4) The results of the expert questionnaire showed that the maximum weight of the indicators affecting the evaluation of the suitability of ecotourism was the satisfaction of tourists (0.120), and the minimum weight was the accommodation facilities (0.002), which illustrated the important role of tourists in the ecotourism development carried out in the nature reserve. (5) Through empirical analysis of numerous cases, the study validates the practicality and effectiveness of the index system and provides scientific guidelines for the suitability of existing nature reserves for further ecotourism development. This contributes to the research theory on the suitability evaluation of ecotourism development and serves as a valuable reference for the future ecotourism development of diverse nature reserves.