In order to research on the function of horizontal collision avoidance of TCAS Ⅳ, which is the next generation of Traffic alert Collision Avoidance System, the models for horizontal conflict detection and collision avoidance are established by using the method of projected horizontal relative distance. The calculating formulae for the projected horizontal miss distance and reserved time to the closet point of approach are derived from the model according to the Galilean principle of relativity. The modeling approach and simulation system for the model are illuminated. The test scheme for the function of horizontal collision avoidance model is provided. The simulation result shows that the model can be used to select reasonable horizontal maneuver measures and indicate the degrees to turn right or left to avoid midair collision.