The invention relates to a fixing device (10) of a down light (100). The fixing device (10) of the down light (100) is characterized by comprising a support (2) and a fastening structure (3), wherein the support (2) is connected to a down light main body (1) and the fastening structure (3) is installed on the support (2). The fastening structure (3) which is opposite to the support (2) can move between an installing position and a fastening position, wherein the installing position can be retractile opposite to the support (2) and the fastening position can extend out and tightly clamp a ceiling opposite to the support (2).The fixing device (10) can bear down lights which are big in weights.