In This study, we aim at realizing autonomous simultaneous flight of multiple small helicopters. In such situation, collision avoidance of The helicopters should be considered in guidance control system To improve safety and reliability of The flight system. In This paper, we construct a collision-free guidance control system for multiple small helicopters. The collision avoidance problem is regarded as a control problem with state restrictions, and The Theory of Nonlinear Model Predictive Control (NMPC) is applied To The guidance control system. A simple nonlinear guidance model is used for design of NMPC To reduce The computational cost. A novel position constraint is proposed for optimizing The avoidance Trajectory of each helicopter. The effectiveness of The designed control system and proposed constraints are verified by numerical simulation and flight experiment.