It had not been explored which real-time magnitude can meet the requirement of the simulation object in computer simulation system. Taking time delay into consideration, the influence of time delay on the stability of the low-altitude and low-speed small Unmanned Aircraft Systems (UAS) flight control system had been analyzed and the real-time magnitude of the miniature hardware-in-loop (HIL) simulation system was determined. Then a miniature HIL simulation system of the UAS flight control system was designed to prove the real-time magnitude feasibility. The result shows the real-time magnitude by time delay analysis can meet the requirements of flight control system.