Abstract: Watershed ecological security is strongly associated with the aquatic ecological status of the upper mountainous area. The present study aimed to assess the watershed ecological security status of the mountainous area under the PSFR (Pressure-State-Function-Response) assessment framework. An evaluation index system was established according to the watershed characteristics, which included four project hierarchical layers, i.e., aquatic ecological pressure, aquatic ecological state, ecological function, and social response, 11 component layers and 23 evaluation indexes. This index system was applied to evaluate the watershed ecological security status of the mountainous area (35 sub-watersheds) in the Taizi River Basin, Liaoning Province. Our results showed that the aquatic ecological status of the study area could be classified into three groups: insecure, general secure and secure, no very insecure and very secure status. Nine sub-watersheds were at the insecure ecological status, accounting for 25.7% of the total sub-watersheds, whereas 22 sub-watersheds were at the general secure state, representing 62.9% of the study area. In contrast, only four sub-watersheds were grouped at the secure status. Furthermore, agricultural activity was identified as the most significant factor responsible for the aquatic ecological security of mountainous area in the Taizi River Basin. Habitat degradation, including water quality deterioration and habitat loss, significantly reduced the ecological functions of the Taizi River Basin, and decreases in rare and peculiar species and biodiversity also posed a threat to the ecological integrity of the study region. Our results could be applied to diagnose the major factors affecting aquatic ecological security, and provide information for effective ecological restoration.