A precision rapeseed seeder with continuous operation often faces the problem of inconsistent sowing quantity with the target seeding rate due to fluctuations in operating speed. To address this issue, a control system for precise rapeseed seeder, based on “RTK speed measurement + seed drop detection + real-time motor control,” was designed. Key parameters affecting the speed control performance were analyzed, and a stepper motor speed control model was established. Static speed drift tests were conducted by using the ZED-F9P GNSS module with the GNSSToolKitLite software. The analysis determined that the static speed drift of the module followed a normal distribution with a mean of 0.054 m/s and a variance of 0.029 m2/s2. The performance tests with an automatic seed particle counter and the JPZS-16 seeding performance test bench determined that the average absolute error between the seeding quantity detection value of the positive and negative pressure seeders and the actual value was less than 10% when the rotation speed was ranged from 5 r/min to 60 r/min. The speed control models for different seeders were determined through bench tests, and the functional relationship between the optimal operating speed range of different seeders and the motor speed with the target seeding rate was analyzed. The adaptability bench tests for different seeders demonstrated that when the operating speed was ranged from 3 km/h to 12 km/h, the total displacement coefficient of variation for the system equipped with a high-speed mechanical centrifugal seeder and a combined air-assisted seeder for rapeseed and wheat was less than 1.21%. Field tests showed that when the operating speed was ranged from 5 km/h to 12 km/h, the total displacement coefficient of variation for the system equipped with a high-speed mechanical centrifugal seeder and a furrow press wheel seeder was less than 9.7%, meeting the requirements of variable-rate seeding. This system can serve as a reference for improving the structure of precision rapeseed seeding control systems.