Abstract: In order to understand the hydraulic characteristics of rigid cascade, the static characteristic roots were obtained by solving the perturbation equations of rigid cascades which have sonic orifice on the heavy fraction pipes. The rigid cascades only have 2 static characteristic roots. Z₁ is equal to 1, and Z₂ is the main static characteristic root and can be used to predict the stability of the cascade. When centrifuge works at rated condition, the main static characteristic root is greater than 1. If any centrifuge is in a stage fail, the fluid parameters can be obtained by solving perturbation equations. The analytic solution can be used to predict the changing trend of fluid parameters if the cascade is disturbed. Although the main static characteristic root is greater than 1, as the disturbances propagate to the waste end, the rigid cascade can maintain the stability.