A shared key management method for a Supervisory Control And Data Acquisition (SCADA) system in which a master terminal unit (MTU), a plurality of sub master terminal units (SUB-MTUs), and a plurality of remote terminal units (RTUs) are configured in a sequential hierarchy, is provided. The method includes: (a) at the MTU, generating a plurality of secret keys and respectively allocating the secret keys to the RTUs; (b) at the MTU, generating a group key in a tree structure, wherein a leaf node of the tree structure corresponds to each RTU, a parent node of a node corresponding to an RTU corresponds to a SUB-RTU to which the RTU is connected, a shared key of each node of the group key is generated by hashing shared keys of all child nodes, and a shared key of a leaf node of the group key is set as a secret key of the RTU; (c) at the RTU or the SUM-MTU, receiving and storing shared keys of every node from a node corresponding to itself to a root node; (d) when the RTU or the SUM-MTU is added or deleted, at the MTU, generating shared keys of nodes along a path from a node corresponding to the added or deleted terminal unit to the root node again; and (e) at the RTU or the SUB-MTU, receiving and storing the generated shared keys. According to the key management method for the SCADA system described above, in the case of encrypting and broadcasting or multicasting a message, a computation amount can be reduced.