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Considering the problem that the large range of land slope changes in rapeseed planting area leads to the seed feeding quantity insufficient stability of the seed feeding device of air-assisted centralized metering device of the planter, a prototype of seed feeding device was developed to improve the stability of seed feeding quantity in slope land planting based on adjusting spring to adjust the distance between seed cleaning brush and circular arc type-hole wheel for controlling feeding and cleaning seed quantity. The working process and principle of the seed feeding device were elaborated. The curvilinear equation of the circular arc type-hole, main arc deflection angle, and structural parameters of seed quantity regulating mechanism was determined. The mechanical relationship between seed quantity regulating mechanism and type-hole wheel was analyzed. Using the intelligent planting machine test platform, the influence experiment of land slope on seed feeding performance was carried out. The influence of seed cleaning brush thickness, adjusting spring effective turns, adjusting plate thickness, and main arc deflection angle on stability of seed feeding quantity in land slope was tested with quadratic rotation orthogonal combination test. The experiment results indicated that using the main objective method in multi-objective optimization, when the seed cleaning brush thickness was 13mm, the adjusting spring effective turns was 82.5, the adjusting plate thickness was 7.8mm, and the main arc deflection angle was 7.7°, the variation of seed feeding rate at 5° tilt was 2.32% compared with no tilt, the variation of seed feeding rate at -5° tilt was 4.29% compared with no tilt, the variation coefficient of seed feeding rate stability was no higher than 0.52%, and the seed feeding stability was better. The bench test results showed that on the relatively flat land with variation of seed feeding rate in the state of oscillation in range of -5° to 5° relative to no tilt was no higher than 1.6%, the variation coefficient of seed feeding rate stability was no higher than 0.86% under the optimum combination of parameters. It could meet the requirement of seed feeding quantity stability in the land slope field planting.