The treatment and prevention of water bloom play a significant role in efficiently controlling water pollution，and the key to solve this problem is to understand formation mechanism of water bloom. Based on a further analysis for the formation process of cyanobacteria bloom in urban lake reservoir，and the whole process is simulated in laboratory，orthogonal experiment and rough set theory are combined to get key factors (DO，TP，TN，temperature and illumination)that can impact formation of cyanobacteria bloom and used as input parameters in establishment of mechanism model for the formation of cyanobacteria bloom in urban lake reservoir.Chlorophyll a is adopted as a characterization index for the formation of cyanobacteria bloom，and based on mechanism of interaction，a model of the process applied to describe cyanobacteria bloom formation in urban lake reservoir is structured.Meanwhile，the parameters involved in this model，such as Gmax，Is，∂，K， Kab， Dmax， mp，can be estimated and nonlinearly optimized by improved PSO(particle swarm optimization)，which can highly enhance the model applicability and validation.The method provides an efficient approach for further study on formation mechanism of cyanobacteria bloom.