Abstract: Abstract: China is rich in crop straw resources, and annual straw output has reached about 700 million t. Due to fluffy and dispersed properties, baling of straw is very necessary for its further utilization. Because round balers don't have the knotter of square balers, the structure of round balers is relatively simple, and it is widely used both at home and abroad. Small and medium-sized round balers are widely used in the rural areas in China. At present, there are some problems when the straw bales are harvested by small and medium-sized round balers developed in China, and the unreasonable working way of the rope-winding mechanism is one of main reasons. In view of the straw bales harvested by small and medium round balers, easy to expand and loosen, a rope-winding mechanism was designed for the steel-roller round baler. On the basis of the measurement of mechanical property of straw bale and the analysis of influencing factors of straw bale expansion deformation, main structural parameters of the rope-winding mechanism, including wheel radius, installation distance between sprocket center and wall side of baling room and circle number of rope at the end of straw bale, were decided as experimental factors, and expansion rate of straw bales and consumption of ropes were selected as evaluation indices. Experiments were done by the round baler equipped with the rope-winding mechanism. The rope-winding mechanism was able to work smoothly in the experiment. According to experimental results, there were certain interactions among circle number, wheel radius and installation distance. Variance analysis was carried out respectively on the regression models of 2 evaluation indices by the Design Expert software. Known from the analysis of variance, the P values of 2 response surface models were all less than 0.0001, and the lack-of-fits were all non-significant, which demonstrated the effect of factors was outstanding and the model was appropriate. The coefficient of determination (R2) for the models were 0.96 and 0.98 respectively, which indicated that the proposed model was agreeable with the practical condition, and thus the proposed model could be used to predict the change of expansion rate of straw bales and consumption of ropes. The results showed that the contributing rate of each factor on expansion rate of straw bales was mounting distance, radius of guiding wheel, and circle number respectively; the order of contributing rate for each factor on the consumption of ropes from high to low was radius of guiding wheel, mounting distance, and circle number. When the circle number was 2 circles, wheel radius was 70 mm, and mounting distance was 62 mm, the optimal evaluation indices were achieved as follows: expansion rate of straw bale was 4.42%, and consumption of rope was 2.93 kg/t. The experimental results can provide a reference for the design and development of rope-winding mechanism of round balers.