The tracking, telemetry and command (TT&C) system is a phased mission system. In this research, the constraint conditions and the allocation objective were discussed, and the mission reliability allocation model was built. The mission reliability allocation of TT&C system is a kind of combined optimization problem with complicated constraints, and a velocity controllable particle swarm optimization (VCPSO) was proposed for this problem. In order to overcome the local convergence of particle swarm optimization (PSO), velocity direction controllable regulation and velocity scale controllable regulation were introduced to increase the population diversity. The velocity iterative strategy in PSO was improved based on the two regulations. By simulation, it shows that the VCPSO can reach more optimal results and convergence in solving the mission reliability allocation of spaceflight TT&C system.