Infrared decoy is a common infrared countermeasures weapon, which has a significant jamming effect on infrared-guided weapons. Among many kinds of infrared decoys, the method of releasing some composition to generate a jamming radiation source is still widely used in practice. In the development and manufacturing process of this kind of decoy, the particle radius of composition has a great influence on decoy＇s radiation characteristics. To study the influence of the particle radius of composition, in this paper, the structure and working principle of infrared decoy were analyzed, the infrared characteristic model of decoy based on discrete phase model (DPM model) was built, the relationship between particle radius of infrared decoy and radiation characteristics was summarized. Finally, through fitting simulation data base on the least square method, the optimal particle radius of 3.72 mm was found while infrared decoy had maximum radiation intensity. This method lays a research foundation for the particle radius of composition of infrared decoy, and also has great guiding value in the field of decoy＇s research and simulation.