Abstract: Abstract: The spray seeding machine use the principle of pneumatic conveying to make the seeds spray through the spray cylinder. The spray cylinder is doing a 0°-180° circumference swing in a uniform linear motion complying with the machine. So, the trajectory of the seed falling on the ground is trochoid. In this way of seed spraying, some areas are sprayed with seeds repeatedly while other areas are missed completely. This seriously affects the uniformity of the seeding. The area which is seeded repeatedly and that which is missed are affected by the sprayer's seeding width, the moving speed of the spray seeding machine, and the swing frequency of the spray cylinder. The swing frequency of the spray cylinder is the main factor, while the other two factors are restricted. So we take the smallest total area of repeated seeding and missed seeding as the objective function to optimize the spray cylinder swing frequency by Matlab GUI. Thus, the program reads the parameters from the text box and calculates the repeated and missed areas and the total area using the numerical integration method and saves them in a matrix, each corresponding to the different swing frequencies. Then the minimum total area and the corresponding swing frequency is found from the matrix. The result is that the optimal swing frequency is 9.5 min-1 when the moving speed is 3 m/s, the spray's outside radius is 20 m and the inside radius is 9 m. The program is correct and workable after the experimental verification. So we calculated the optimal swing frequencies corresponding to different moving speeds. This study provides a theoretical basis for spray seeding machine designing.