In order to determine the key parameters of the flattening component on combined cultivating implement, the field experiment was conducted by using the flattening device. The three factors and three levels orthogonal regressive tests were designed in the experiment with working velocity, diameter of the field-ditch filling disc, angle of the field-ditch filling disc as influence factors and the soil-breaking rate as response index. The regression mathematics models between response index and influence factors were established and the parameters were optimized through Design-Expert. The optimal combination was that working velocity of 10.63km/h, diameter of the field-ditch filling disc of 450 mm, angle of the field-ditch filling disc of 17.71°. At that time the soil-breaking rate was 91.92%, which could meet the needs of agricultural demands. 