A restraining device for a child safety seat comprising: two attachment assemblies; and a crossbar interconnecting the attachment assemblies; characterized in that each attachment assembly comprises: an attachment unit secured to the crossbar and including a first member and a second member secured to the first member; an attachment member pivotably fastened in the attachment unit and including a first cavity and a second cavity; a spring biased first locking member having one end moveably disposed in the first cavity and including a main body, a locking element slidably secured to the main body, and a first elastic member biased between the main body and the locking element; a second locking member engaging the attachment member; a spring biased moveable member having one end secured to the second locking member; and a spring biased lock pivotably fastened between the first element and the second element and including a lever, a following member, a positioning member extending from the lever through the following member into the moveable member to hold in position, and a second elastic member biased between the following member and the lever. two attachment assemblies; and a crossbar interconnecting the attachment assemblies; characterized in that each attachment assembly comprises: an attachment unit secured to the crossbar and including a first member and a second member secured to the first member; an attachment member pivotably fastened in the attachment unit and including a first cavity and a second cavity; a spring biased first locking member having one end moveably disposed in the first cavity and including a main body, a locking element slidably secured to the main body, and a first elastic member biased between the main body and the locking element; a second locking member engaging the attachment member; a spring biased moveable member having one end secured to the second locking member; and a spring biased lock pivotably fastened between the first element and the second element and including a lever, a following member, a positioning member extending from the lever through the following member into the moveable member to hold in position, and a second elastic member biased between the following member and the lever. an attachment unit secured to the crossbar and including a first member and a second member secured to the first member; an attachment member pivotably fastened in the attachment unit and including a first cavity and a second cavity; a spring biased first locking member having one end moveably disposed in the first cavity and including a main body, a locking element slidably secured to the main body, and a first elastic member biased between the main body and the locking element; a second locking member engaging the attachment member; a spring biased moveable member having one end secured to the second locking member; and a spring biased lock pivotably fastened between the first element and the second element and including a lever, a following member, a positioning member extending from the lever through the following member into the moveable member to hold in position, and a second elastic member biased between the following member and the lever.