The invention discloses a die for riveting an automobile tension pulley assembly, comprising an upper die device, a guide sleeve, a riveting head and a lower die, wherein the guide sleeve is sleeved at the lower end of the upper die device and is connected with the upper die device through a compression spring; the riveting head is arranged in the guide sleeve and is connected with the lower end of the upper die device; the lower die is arranged opposite to the upper die device. During riveting, the upper die device and the lower die are fixed on a hydraulic press, the guide sleeve is connected with the lower end of the upper die device through the compression spring, and the riveting head is arranged in the guide sleeve. In the riveting process, the upper die device moves downwards, and the end surface of the guide sleeve is in contact with a regulating flat plate firstly to form prepressing so as to ensure that the regulating flat plate is in close fit with a lining, thereby solvingthe problem that the regulating flat plate cannot be in close fit with the lining because one side of the regulating flat plate tilts; and along with the continuous downward moving of an upper die, the compression spring contracts, and the riveting head extends out of the guide sleeve for riveting. According to the invention, the problem that the verticality of the tension pulley assembly cannot meet the requirement due to gaps generated during riveting the tension pulley is effectively solved, and the assembling precision is improved.