The embodiment of the invention discloses a method for acquiring a safety assessment parameter of a transmission network. The method comprises the following steps: repeatedly executing the following operations: acquiring frequency of occurrence of each network element in a first set; deleting a direct-connected network element corresponding to the network element with the frequency of occurrence of 1 from the first set; acquiring the network element number in the first set until the network element with the frequency of occurrence of 1 cannot be searched from the first set; and determining a ring formation rate of the transmission network according to the network element number in the first set and the network element number of the transmission network. The embodiment of the invention further discloses a device for acquiring the safety assessment parameter of the transmission network.