This paper concerned the optimal design of a multirate sampled-data control system based on robust stability. For a system with a given F, a new condition for robust stability test of uncertain system was developed by extending Lyapunov upper bound method. For a system with an unknown uncertainty bound, the robust stability radius (RSR) was defined. The RSR calculation problem could be converted to a mathematical process of solving an equation by analyzing the robust stability test condition proposed. The equation was solved using numerical mathematical methods to get the analytical solution as RSR. As a function of F, the F optimization design could be achieved by a general optimization method to get the biggest RSR. With this F, the system could remain stable with the uncertain parameters varying set being the largest. Finally, a simple example was provided to test the feasibility of the proposed method.