Abstract: Topdressing has been widely used to supplement the basic fertilizer in the middle and later growing stages of rice paddy fields. Broadcasting is one of the fertilizer applications to meet the nutritional needs of rice. The commonly used broadcasted fertilizer may flow with the surface water in rice production, resulting in environmental pollution, fertilizer loss and a low utilization rate. And a furrow opener of the most deep fertilization machine can result in the blockage of the working parts and damage to plant roots. In this study, a Super-large Fertilizer Granule (SFG) accelerator was designed for deep fertilization in the paddy field, in order to reduce the fertilizer loss with a high utilization rate. The high speed of the SFG was achieved in the release and long-term effectiveness of SFG. The high-pressure airflow was formed at the twin spiral inlet within an approximately closed space in the acceleration tube above the SFG. The SFG was accelerated to avoid the collision of the SFG on the wall of the accelerator tube as well, in order to reduce the speed loss of the SFG; The airflow was diffused at the outlet to reduce its speed and the impact on the soil, in order to improve the stability of fertilizer position. The structural parameters of the accelerator were determined using the physical parameters of SFG. The working parameters of the accelerator were analyzed to fully meet the requirement of the injection speed for the depth of fertilization. A simulation model was established for the accelerator using the Fluent platform. 6DOF overlapping dynamic grid was selected to simulate the flow field in the accelerating tube in the fluid domain. The single-factor experiments and three-factor three-level Box Behnken combination experiments were conducted with the inlet airflow speed, inlet spiral angle, and acceleration tube diameter as the experimental factors. A systematic investigation was implemented to explore the effects of the factors on the injection speed of fertilizer and the diffusion rate of outlet airflow. The regression model and response surface analysis of the test indexes were established by multifactor experimental analysis. The results showed the optimal working parameters were achieved in the accelerator: inlet airflow speed of 47 m/s, acceleration tube diameter of 21 mm, and spiral inlet angle of 43°, when the weight of the accelerator fertilizer injection speed was 0.6, and the weight of the outlet airflow diffusion rate was 0.4. The bench test was carried out using the 3D-printed SFG spiral accelerator, where the SFG was developed by National Institute of Biochar, Shenyang Agricultural University. The average fertilizer ejection velocity was measured to be 12.61 m/s, the average diffusion rate of outlet airflow was 85.5%, and the average depth of fertilizer into the soil was 4.68 cm, thus meeting the requirements for the deep fertilization of rice. There was a small relative error between the predicted value from the multi-factor regression and the experimental, indicating the more accurate optimization of the simulation test. This finding can provide a promising basis for designing the pneumatic machines of deep application for the super-large fertilizer granule in paddy fields.