Semi-arid closed lakes are critical nodes where ecological, climate, and human factors intertwine. Their multifunctionality plays an important supporting role in regional sustainable development. This study focuses on the ecological function maintenance of closed lakes in semi-arid regions, with Lake Daihai, a typical inland closed saline lake in the Mongolian Plateau, as the research subject. Given the ongoing deterioration of its water environment, this study conducts a systematic analysis based on continuous monitoring data from 2020 to 2024. Using Principal Component Analysis/Factor Analysis (PCA/FA), five key water quality parameters—dissolved oxygen (DO), chemical oxygen demand (CODCr), total nitrogen (TN), total phosphorus (TP), and salinity (Sal)—were selected from the multidimensional water quality dataset. The spatiotemporal variation patterns of the water environment parameters in Lake Daihai were systematically analyzed, and the Water Quality Index (WQI) was used for quantitative assessment. Pollution source apportionment was carried out using a comparative analysis of the Absolute Principal Component Scores-Multiple Linear Regression (APCS-MLR) and Positive Matrix Factorization (PMF) models. The results show that the PMF model exhibited better performance in pollution source identification. The source apportionment results revealed that the main pollution sources in Lake Daihai are as follows, with their contribution rates: rural domestic pollution (23.5%), planktonic endogenous release (22.5%), livestock farming pollution (20.1%), surface runoff pollution (19.6%), and agricultural cultivation pollution (14.4%). This study quantifies the contribution rates of pollution sources in Lake Daihai, providing a theoretical basis for targeted management and ecological restoration of Daihai and similar lakes in semi-arid regions.