Abstract: With a series of advantages, such as fast settling velocity, high biological concentration and low sludge production, granular sludge has attracted extensive attention from researchers at home and abroad since its discovery. To date, there have been a large number of studies on sludge granulation conditions and their influence mechanism, among which the hydraulic condition of reactors is a widely accepted key factor affecting sludge granulation. The existing research results about the impact of hydraulic conditions on the formation and stability of aerobic and anaerobic granular sludge were analyzed, finding that the formation and stability of granular sludge must be controlled within a certain range of hydraulic conditions. However, it is difficult to arrive at a unified result from different research platforms because of the extremely complex factors influencing the hydraulic conditions. Then, methods for controlling and expressing the existing hydraulic conditions were summarized to analyze the mechanisms of different controlling methods influencing the hydraulic conditions from the microscopic level, as well as the problems of the existing hydraulic condition expressions, such as being relatively macro- or difficult to display local characteristics. Fortunately, computational hydrodynamic methods, recently introduced to study reactor hydrodynamic conditions, will play a significant role in accurately expressing the hydraulic conditions for sludge granulation. By retrospect, summarization, analysis and development prediction for the influence mechanisms and control and expression methods of hydraulic conditions in the sludge granulation process, references and guidance for subsequent research are expected.