The invention provides a lathe airtightness detecting device, and belongs to the technical field of machines. The lathe airtightness detecting device solves the problem in an existing lathe airtightness detecting device that, a clamping point and an airtightness detecting point of a workpiece are centralized together, and chip discharge is hindered. The lathe airtightness detecting device comprises a locating base coaxially arranged at the front end of a chuck, several supporting columns of which the quantity is equal to that of clamping pawls on the chuck are arranged at the edge of the fixing base, are uniformly distributed and extend forwards in the axial direction of the locating base, the locating base is internally provided with a main air channel, the supporting columns are internally provided with branch air channels communicated with the main air channel, an annular offset base is arranged at the front end of the several supporting columns, a locating surface used for locatinga step surface of the workpiece is arranged at the front end of the offset base and is provided with air vents opposite to a technology point of the workpiece, the quantity of the air vents is equalto that of the supporting columns, and the air vents are communicated with the branch air vents, corresponding to the air vents, in the supporting columns. The lathe airtightness detecting device hasthe advantages of facilitating chip discharge.