The utility model provides a derailleur gas tightness detection device belongs to mechanical technical field. It has been solved current derailleur gas tightness detect process flow and has had the problem that the defect leads to nonconforming product inflow customer. This derailleur gas tightness detection device, including the frame of locating the assembly line lateral part, locate in the frame the leak test appearance and with the leak test frock that the leak test appearance is connected, be equipped with the pallet who is used for the centre gripping product on the assembly line, be equipped with in the frame with move to the product of frame department correspond the seal unit that sets up and be used for controlling the seal unit to qualified products with the the control unit who covers the mark, the control unit is connected with the leak test appearance. The utility model has the advantages of the mistake proofing is effectual, carries out on -line measuring, saves and hangs the step into the work table with the product, reduces and detects the process, guarantees product quality simultaneously, improves production efficiency and reduction in production cost.