King grass has strong tiller ability, with high stalk fiber content and high moisture content. Based on the crushing device of forage harvester, a roller crushing device suitable for harvesting king grass was designed according to the demand of harvesting. The main structure and parameters were determined through theoretical analysis. The crushing knife of the device adopted a four-row symmetrical arrangement, which could effectively reduce the fatigue damage caused by the alternating introduction. A single factor test and a combined quadratic orthogonal rotation test were designed to explore the influence of rotating speed of feeding roller and the crushing roller on the evaluation indicators: stalk crushing pass rate and average length. Response surface analysis, regression analysis and target optimization were carried out on the results by Design-Expert. Regression equations and optimal combinations of parameters between the test factors and evaluation indicators were obtained. The results of single factor test revealed that the rotating speed ranges of the feeding roller and the crushing roller were 380~480r/min and 750~950r/min, respectively. Quadratic rotation orthogonal test and target optimization obtained the optimal combination as follows: the feed roller rotating speed was 416.5r/min, the crushing roller rotating speed was 950r/min, the qualified rate of stalk crushing was 98.30%, and the average length of broken stalks was 29.04mm. The bench test verification and the field test revealed that it had a uniform crushing effect, a high pass rate of stalk crushing and the average length met the requirements, which met the design requirements of the roller crushing device.