Abstract: In order to lower the percentage of oversize strips after threshing, five combinations of parameters were tested for the first stage thresher, including clearance between teeth and frame basket, the arrangement of teeth and the shape of frame openings. The results showed that:1) The threshing efficiency and the size and shape distribution of threshed strips differed between different combinations of parameters. 2) The preferable combination of parameters included denser rhombic frame basket openings and mixed rectangular and trapezoidal teeth arranged in Y-shape. 3) Under the preferable conditions, the percentage of large-sized strips decreased by 6.55 percentage points, that of medium-sized strips increased by 6.35 percentage points, while the percentages of large-and medium-sized strips and small-sized strips kept unchanged. The amount of large-sized strips with the length to width ratio of 1.2-1.5 increased obviously (by 9.07%), and the amount of large-and medium-sized strips with the length to width ratio of 2-3 decreased obviously (by 9.59% and 7.57%, respectively). The parameter-modified first stage thresher effectively promotes threshing efficiency and generates better strips.