An initial velocity accelerating tube includes an outer pipe. Two ends of the outer pipe are connected with a stop member and a connection member, respectively. An inner pipe is provided in the outer pipe. Two ends of the inner pipe are inserted in the stop member and the connection member, respectively. When the initial velocity accelerating tube is mounted on a toy gun, one end of the inner pipe is against the front end of the barrel of the toy gun by the elastic force of the elastic member so as to tightly connect the barrel, providing an airtight effect. The barrel can be lengthened. To have a shoot with the long-barreled gun, the bullet can have a longer route to accelerate for getting a higher initial velocity.