The simplified radiance model of midcourse ballistic target and the infrared focal plane model based on point spread function and target tail were presented. By integrating the calculations for target space position and the position in 2D focal plane projected from 3-D space， the imaging of midcourse ballistic object target complex in space-based infrared sensor was simulated. By analysing the simulation results， it was found out that the midcourse ballistic object target complex would give rise to new challenges for detection and tracking techniques of space-based infrared sensor. The methods to solve these challenges were given here.