The present invention relates to a safety device for a quick coupler and, more particularly, to a safety device for a quick coupler, the safety device having an improved structure that is configured to improve the structure of a locking device, the locking device preventing the fixing hook from being arbitrarily released from a locking engagement, by using a safety plate that is laterally moved in a sliding manner by an elastic force of a DIE spring after a fixing hook and a first attachment pin are assembled together.