With the Thoma's assumption, the critical sectional area for flow stability of the downstream throttled surge tank is derived theoretically. The effects of the velocity head and momentum exchange at the Tee junction between the surge tank riser and the tail water tunnel on the critical sectional area of the downstream tank have been analyzed using the Gardel's head loss formula. The study shows that the velocity head has an unfavorable effect on the water level stability in the downstream surge tank while the effect of the momentum exchange is positive. Two effects counteract. Thus the critical sectional area of a downstream throttled surge tank is close to that of a corresponding simple surge tank. However, if the size and shape of a riser are optimized, the area of the downstream surge tank may be further reduced. It is also demonstrated that a downstream throttled surge tank can reduce electricity loss and meet the requirements of stability easily in large flow oscillations compared to a simple downstream tank.