In engineering construction, waste mud often contains an extremely high water content that can significantly impact the environment. Addressing the issue of dehydration is therefore a primary concern. To address the problem of high water content mud water separation, this study selected three types of flocculants CaCl₂, anionic polyacrylamide (APAM) and poly aluminum chloride (PAC), and then designed a 3-factor and 4-level orthogonal experiment using the comprehensive balance method. Indicators such as the 48-hour dehydration amount and pH value of the supernatant were selected, and range analysis and variance analysis were conducted to find the optimal combination of composite flocculants. The flocculation mechanism was also discussed through orthogonal test results and SEM microanalysis. The experimental results showed that dividing the dehydration rate curve over time into high, medium, and low dehydration zones, as well as fast and slow dehydration stages, led to a decrease in the overall mud dehydration rate. However, the high dehydration zone group had a higher dehydration rate at 12 h, which met the requirements for fast dehydration. Additionally, the three flocculants showed some synergistic effect. Through range analysis of variance analysis orthogonal tests, the optimal combination of composite flocculants was found to be 0.25 g CaCl₂ + 0.12 g APAM + 0.17 g PAC per 100 g total mass of slurry. Based on the mechanism analysis, the flocculation process of composite flocculants was divided into five stages: charge neutralization stage, bridging stage, adsorption agglomeration stage, network sedimentation stage, and solidification enhancement stage.