In response to the practical problem of the lack of light and simple integrated planter’ seeding device and fertilizing device for rapeseed in small fields such as hilly and mountainous areas, a centralized seeding-fertilizing device for rapeseed was developed. The working process of the centralized seeding-fertilizing device for rapeseed was clarified. The main structural parameters of the seeding cylinder and fertilizing cylinder were determined. The influence of the fertilizer layer control board position on the matching of seeding and fertilizing quantity was analyzed. An experiment was conducted using EDEM simulation to investigate the seeding and fertilizing performance of the centralized seeding-fertilizing device for rapeseed. A model was established to determine the relationship between the fertilizer layer control board position and the fertilizing quantity. The experimental results showed that when the seed-fertilizer cylinder rotating speed was 20~50 r/min, the seed-fertilizer cylinder rotating speed was linearly related to the seeding quantity and fertilizing quantity. When the angle between the fertilizer layer control board and the vertical direction increased within 22°~38°, the fertilizing rate was gradually increased between 1 709.63 g/min and 2 484.31 g/min. The DEM-CFD coupling simulation was applied to analyze the influence of auxiliary conveying airflow on the seed velocity and force inside the seed guiding tube. The results indicated that when the inclination angle between the seed guiding tube and the vertical plane was 10°~70°, the auxiliary conveying airflow could achieve accelerated seed discharge through the seed guiding tube and avoid seed retention. The results of the bench validation test showed that when the seed-fertilizer cylinder rotating speed was 20~50 r/min, the stability variation coefficient of the total seeding quantity was not higher than 0.92%, the consistency variation coefficient of each row of seeding quantity was not higher than 3.64%, the seed damage rate was not higher than 0.04%, the stability variation coefficient of the total fertilizing quantity was not higher than 1.21%, and the consistency variation coefficient of each row of fertilizing was not higher than 4.41%. As the seeding rapeseed was fixed, the angle between the fertilizer layer control board and the vertical direction could be adjusted to meet different fertilizing requirements. The field test results indicated that the variation coefficient for the consistency of the number of plants in each row of rapeseed was 9.25%, and the variation coefficient for the stability of the total number of plants was 7.16%, which met the requirements for precision seeding for rapeseed.