Abstract: In rice-wheat double-cropping area, the top soil is compactible and the soil disturbance frequency is relatively lower, which eventually results in soil hardening due to the implementation of lacking tillage or no tillage. In order to increase soil disturbance frequency and improve soil environment based on conservation tillage, and solve excessive straw yield in full amount of straw returning under high-yield production, a new straw returning model named straw-returning-to-ditch was proposed. According to operation characteristics of this model, a multifunction machine combining harvester and ditch opener was designed. This machine could complete harvesting, ditching and stalk-disposing at one time, and realize multiple assignments with high efficiency. The ditching device of the multifunctional machine was biased in line with the unilateral grass outlet, which should however be aligned with the grass outlet to ensure the rice straw to be collected into the ditch reliably. So, after long-term practice, the assignment of the ditching device led to unilateral bearing, poor straight walking performance and running deviation for this machine, which was difficult to manipulate. Meanwhile, the machine still had some threshing problems, for example, under threshing and high loss rate in the process of harvesting. Therefore, based on the principle of transfer guide function and secondary threshing of the lateral cylinder, an assisted threshing cylinder that was located at the rear of the main threshing cylinder was equipped, which constituted a double threshing cylinder structure. And the grass inlet was connected with the main threshing cylinder. The double threshing cylinder structure could effectively solve the problem of the machine unilateral bearing and low harvest quality through changing the position of grass outlet, making the ditching device located in the middle surface of the track to eliminate deflecting force, and increasing the threshing time to improve harvesting quality. Combined with the characteristics and the available space in the rear of the main threshing cylinder, the overall length and diameter of the assisted threshing cylinder were 855 and 452 mm respectively, the diversion angle of cylinder end cover plate was 18°, and the angular speed was 1 350 r/min. The modified grass outlet and crawler surface were kept on the same vertical plane, and the distance between their centers was 573 mm. The performance test showed that the walking position deflections of modified machine under 3 different working speeds (low speed 0.38 m/s, medium speed 0.56 m/s and high speed 0.74 m/s) were reduced by 93.9%, 94.4% and 93.3%, respectively. Furthermore, the straight walking stability and control performance were significantly improved. In the harvest process, the total loss rates of wheat and rice were decreased by 20.9% and 11.8%, respectively, while the impurity rates were decreased by 45.7% and 21.4% respectively. The comprehensive performance of rice threshing was enhanced although the broken rate was increased by 7.4%; and meanwhile the broken rate could be acceptable for the use of harvesting machine. The result indicated that the double threshing cylinder structure was feasible for solving the problems of the machine. This research has improved the applicability of the multifunctional machine and provided the reference for the mechanical returning of the collected straw to the ditch.