Abstract: Precision fertilization can greatly contribute to the utilization rate of chemical fertilizers and ecological safety. The amount of chemical fertilizer can also be reduced for the rational demand of agricultural production. Among them, the fertilizer ejector is one of the most important components during precision fertilization. Its performance is of great significance to improve the quality of granular fertilizer application. However, the existing research focuses mainly on the structural optimization of the fertilizer ejector for the better fluidity of fertilizer particles. It still lacks the precision, the lag of fertilizer discharge, and easy clogging of granular fertilizer at the same time. In this study, a variable-diameter pneumatic-driven discharger of precision fertilizer was developed to integrate "discharge opening adjustment + rotational speed control + pneumatic forced discharge". The high accuracy was achieved to mitigate the fertilization hysteresis under varying operational speeds, in order to fully meet the agronomic requirements across different field plots during mechanized direct seeding of rapeseed. A systematic optimization was also carried out on the key parameters of the variable diameter structure, air-fertilizer mixing device, airflow distribution, and fertilizer guiding structure. The discharge tests were also conducted to calibrate the system. A control model was established to correlate the fertilizer discharge rate with the rotation speed and discharge opening. Dual-variable precision fertilization was implemented. The tests demonstrated that the optimal discharge range of 4.27-61.41 g/s was achieved with rotation speeds of 30-120 r/min and openings of 15-25 mm, fully meeting the operational requirements for the rapeseed direct seeding at the speeds of 2.5-12.0 km/h and the fertilization rates of 225-600 kg/hm². There was a high accuracy of fertilizer discharging of 96.30%-98.66%. The coefficient of variation for the consistency and stability of displacement in each row (C_v1) of 2.58%-8.62% and the same industry (C_v2) of 2.81%-6.85% fully met the national standards. Furthermore, the pneumatic-driven discharger eliminated the blockages in the operational range of 3.25-61.25 g/s. Specifically, the fertilizer particle velocity was enhanced by 44%, whereas, the discharge time was reduced by 44%, compared with the non-pneumatic ones. A field test confirmed that the high accuracy of fertilization was achieved over 95.07% via the precise control of opening and rotation speed, particularly under the fertilization rates of 300-600 kg/hm² and the operational speeds of 3.2-12.1 km/h. These findings can provide the technical references to develop the variable-rate fertilization equipment in rapeseed direct seeding.