On the basis of signal flow graph, the method of loop analysis was put forward to study the mechanism of velocity vector instability and its stable control. The signal flow graph of the longitudinal dynamics was established, and the convergence theorem of loop was proved. On this basis, through theoretic analysis, the reasons of velocity vector instability at different levels were obtained, and the velocity and flight path′s instability expressions were obtained too. In addition, it was also shown that the drag-velocity curve, the track angle-velocity curve, the polar curve and the drag-coefficient curve possess mutual backside area and frontside area, and the drag-velocity curve′s slope is proportional to the track angle-velocity curve′ s. The conclusions about the velocity vector′s stability critical condition, convergence expressions and stability mechanism under APCS (approach power compensation system) were obtained, and the comparison results about control performance between velocity constant APCS and angle-of-attack constant APCS were obtained through theoretic analysis and simulation.