A connection device of a cable management arm includes a fixing frame having a protrusion portion, a connection member having a contact portion, and a locking member pivotably connected to the connection member. The locking member includes a locking portion and a resilient portion which contacts the contact portion of the connection member. When the connection member is pivotally connected to the cable management arm, the fixing frame is fixed to a rail. When the connection member is connected to the fixing frame, the resilient portion provides a force to lock the locking portion to the protrusion portion of the fixing frame so as to securely connect the connection member to the fixing frame.