Fodder rapeseed is a good quality feed crop, and it can effectively alleviate the problem of lack of green feed for livestock in winter and spring. Because of high water content and crispy texture of fodder rapeseed, there is a lack of applicable harvesting machinery in Midlower Yangtze River. To solve the problems, a kind of cylindertype chopping device was designed. The structure and operation parameters of chopping device were analyzed through material properties and the requirements of cutting and throwing operations. Single factor experiment and rotation regression design were carried out to study the effects of feeding roller and cutterhead rotational speed on length qualified rate and power. The regression equation of the relationship between feeding roller and cutterhead rotational speed, and the length qualified rate and power was obtained. The results showed that the length qualified rate was better when the feeding roller rotational speed was 400~550r/min and the cutterhead rotational speed was 600~800r/min. The optimum operation condition was: the feeding roller rotational speed was 496.17r/min, the cutterhead rotational speed was 709.14r/min. Under this condition, the length qualified rate was 91.16% by calculation. What was more, the field experiment of fodder rapeseed harvester was conducted. It turned out that the cylinder-type chopping device worked well and the length qualified rate met the requirements of feeding. The results could provide support for optimization of the cylinder-type chopping device and the improvement of the overall structure for fodder rapeseed harvester.