A straw chopper cum spreader was developed to deal with the problems of unadjustable chopped straw spreading uniformity and width. The machine which was mainly composed of a curved shell, transmission system, eight guide vanes, a deflector, a guide vane regulating device, a running gear, a hood and a straw chopping device can realize the function of crop straw chopping and chopped straw spreading. The front casing of the shell was a logarithmic spiral line surface, and the equation of the front casing was built. The comparison between curved type shell and break line type shell was made by using Fluent software, the results of which showed that the curved type shell could improve the flowability of straw in the shell. A row of stationary blades was mounted to support the chopping process of the combined flails, which could help to reduce length of the chopped straw. The distance between the stationary blade and combined flail was determined according to the chopping quality. The fan-shaped blades which were mounted on both side of the roller increased the fluid flow rate, and improved the straw feeding ability. The guide vane can guide the flow of chopped straw at the outlet of the shell, which improved the spreading uniformity of the chopped straw. The guide vane regulating device can control the spreading width, velocity and uniformity of the chopped straw by adjusting the guide vanes position on the deflector. The structural parameters of the guide vane regulating device were determined according to the features of mechanisms. Field experiment results showed that with the forward speed of 1.8m/s, straw moisture of 78.4%, and PTO rotation speed of 540r/min in the unharvested cornfield, the machine could achieve a qualification rate of straw chopping of 90.01%, spreading width of 2223.3mm, average stubble height of 62.0mm, and spreading unevenness of 22.95%. The performance indexes of the machine satisfied the agronomic requirements of returning straw onto soil surface. The research can provide some references for the design of new chopper with good straw chopping and spreading ability.