Abstract: IN RECENT YEARS, WITH THE INTENSIFICATION OF GLOBAL CLIMATE CHANGE AND HUMAN ACTIVITIES, ABNORMAL PROLIFERATION OF FILAMENTOUS CYANOBACTERIA HAS OCCURRED FREQUENTLY WORLDWIDE, BECOMING A MAJOR THREAT TO WATER QUALITY SAFETY IN LAKES AND RESERVOIRS. AS THE CORE WATER SOURCE OF THE MIDDLE ROUTE OF THE SOUTH-TO-NORTH WATER DIVERSION PROJECT, THE WATER QUALITY SAFETY OF THE DANJIANGKOU RESERVOIR DIRECTLY AFFECTS THE DRINKING WATER SECURITY OF TENS OF MILLIONS OF PEOPLE ALONG ITS ROUTE. IN THE SUMMER OF 2025, THE DANJIANGKOU RESERVOIR EXPERIENCED FOR THE FIRST TIME A LARGE-SCALE ABNORMAL PROLIFERATION DOMINATED BY FILAMENTOUS CYANOBACTERIA SUCH AS RAPHIDIOPSIS AND LEPTOLYNGBYA, WITH PEAK ALGAL DENSITY EXCEEDING 1×10⁸ CELLS/L. HOWEVER, CHLOROPHYLL-A CONCENTRATION AND BIOMASS REMAINED RELATIVELY LOW, AND NO TYPICAL SURFACE SCUM CHARACTERISTICS OF ALGAL BLOOMS WERE OBSERVED, POSING A CHALLENGE TO EXISTING ALGAL BLOOM ASSESSMENT AND EARLY WARNING SYSTEMS. BASED ON EMERGENCY MONITORING DATA COLLECTED FROM JULY TO SEPTEMBER 2025 AT THE TAOCHA WATER AREA, A KEY AREA OF THE DANJIANGKOU RESERVOIR, AND IN CONJUNCTION WITH DOMESTIC AND INTERNATIONAL RESEARCH ON FILAMENTOUS CYANOBACTERIA, THIS STUDY EXPLORES A RISK EARLY WARNING INDICATOR SYSTEM AND THRESHOLD DETERMINATION METHOD APPLICABLE TO ABNORMAL PROLIFERATION OF FILAMENTOUS CYANOBACTERIA. IT PROPOSES A CORE SET OF EARLY WARNING INDICATORS INCLUDING TOTAL ALGAL DENSITY, CHLOROPHYLL-A CONCENTRATION, AND 2-METHYLISOBORNEOL (MIB) CONTENT, AND ESTABLISHES A CORRESPONDING RELATIONSHIP WITH ACTUAL ALGAL DENSITY THROUGH VOLUMETRIC CONVERSION METHODS. THE AIM IS TO PROVIDE A SCIENTIFIC BASIS FOR THE PREVENTION AND CONTROL OF FILAMENTOUS CYANOBACTERIA RISKS IN THE DANJIANGKOU RESERVOIR AND SIMILAR WATER SOURCE AREAS.