The wide application of sensitive loads increases the customer requirement for power quality. The rapid and efficient assessment of voltage sag is becoming important in voltage sag study. In this paper, a stochastic estimation method of voltage sags for a distribution network based on network propagation property is proposed. First, a voltage sag propagation path is searched based on distribution network topology. With the phase component method, the vertical/horizontal propagation property of voltage sag is derived. Then the voltage sag propagation equation in a distribution network from the fault source to the load side is established according to the path search results. Finally, the area of vulnerability and the estimation index are calculated by combining different fault types and line fault rates. An example analysis of a real distribution network is given to verify validity and superiority of this method. This work is supported by Science and Technology Project of China Southern Power Grid Company (No. GDKJXM- 20180103).