Abstract: To reveal the influence mechanism of water quality on coal slime flotation effect during coal preparation, this study focuses on key water quality parameters such as ion types (Ca²⁺and Al³⁺), ion concentration, pH value, and water hardness. It systematically investigates their action laws on the coal particle-bubble encapsulation angle and verifies the correlation between the encapsulation angle and flotation effect through tests. The research uses high-speed camera technology to capture the collision and adhesion process between coal particles and bubbles, characterizes the interaction intensity between them by measuring the encapsulation angle, and synchronously analyzes the changes in clean coal yield, ash content, and flotation perfection index under different water quality environments. The results show that: with the increase of ion concentration, the coal particle-bubble encapsulation angle shows an upward trend, among which the coking coal particle-bubble encapsulation angle responds most obviously. During the flotation process, a lower ion concentration (< 10⁻⁴ mol/L) should be maintained, at which a higher flotation clean coal yield and qualified ash content can be obtained;within the pH range of 5～9, the encapsulation angle is less affected by pH value, reaching the highest value at pH = 7, and slightly decreasing when pH > 7. The flotation clean coal yield first increases and then decreases with the increase of pH value. When the solution is neutral or weakly alkaline (pH = 7 or 8), a higher flotation clean coal yield and lower clean coal ash content can be obtained;water hardness has a significant impact on the encapsulation angle, which reaches the maximum value when the water hardness is about 400 mg/L. When the flotation water hardness is 300～400 mg/L, the flotation clean coal yield of the three coal types is relatively high, reaching more than 75.00%, with lower clean coal ash content and relatively high flotation perfection index. Therefore, flotation can be carried out in an environment with lower water hardness. This study clarifies the internal mechanism by which water quality parameters affect the flotation performance by regulating the encapsulation angle, providing theoretical support and practical guidance for optimizing coal slime flotation process parameters and improving separation efficiency.