Through stream breaking by stream interrupter installed in the rotary sprinkler, close-in water will be increased and the water distribution will be enhanced. The rotary stream interrupter rotates intermittently under the stream impact, which leads to improve the water uniformity, and at the same time, almost does not reduce the throw distance. The structural size of the diffuser fixed in the stream interrupter plays an important role to the hydraulic performance of the sprinkler. The shortest performance radius R and the affected area angle α produced by the diffuser are put forward as the optimizing indexes in the orthogonal experiment. The key sizes, such as the diffuser width a, the diffuser length interrupting in the stream h and the distance from the outlet nozzle to the diffuser c, are presented as factors. By testing the hydraulic performance influenced by the factors, the relationships among the factors and the performance radius and the affected area angle are obtained, and the optimum structural sizes of the diffuser are also brought forward. The radical water distribution of a rotaty sprinkler is composed of the distributions without the diffuser and with the diffuser by the definite spraying frequency. According to above meationed law and the concept that the ideal shape of the final water distribution is triangle, the method to calculate the shortest performance radius in the affected area is first built up. 