A device for detecting air tightness of a casing of an automobile reducer comprises a press device, an upper sealing device, a lower sealing device, a gas supply system, a low-pressure leak detectingsystem, a main control system, an extension rod and a pressure head, wherein the extension rod is connected with a piston rod of a cylinder, an axial blind hole is drilled in the center of the extension rod, two radial holes of a compressed air inlet hole and a low-pressure leaking instrument connecting hole that are communicated with the blind hole are drilled on the rod part of the extension rod, the pressure head is fixed on a flange plate at the lower end of the extension rod by a bolt, a through hole for passing of the compressed air is opened in the center of the pressure head, a ring slot at the lower end of the pressure head is sleeved with a sealing ring, the ring slot is sealed with the end face of an oil sealing hole at the upper end of the casing of the reducer under the pressure of the cylinder, the lower sealing device is installed at the central opening of the equipment platform and is a barrel-shaped sealing cylinder, the upper end of the cylinder body is open and sealed with the flange surface at the lower end of the casing of the reducer, and the lower sealing cylinder includes an O-shaped sealing cylinder and a narrow circular micro-bump sealing cylinder. A device for detecting air tightness of a casing of an automobile reducer provided by the invention has stable and reliable sealing performance, and high applicability, and only needs to replace the pressurehead and the sealing cylinder to check casings of different reducers, thereby saving production and use costs.