Nondestructive testing device for quality of existing railway tunnel lining

The invention discloses a nondestructive testing device for the quality of an existing railway tunnel lining. The device comprises a testing mechanism, a self-resetting floating mechanism, a linear telescoping mechanism and a pitching mechanism which are sequentially connected, wherein the detection mechanism comprises a hollow profile arm, an arc telescopic arm and a radar antenna unit, the profile arm is in an arc shape matched with that of the tunnel lining, the arc telescopic arm matched with the profile arm is arranged in the profile arm, one end of the arc telescopic arm is located in the profile arm, and the radar antenna unit is arranged at the other end; the self-resetting floating mechanism comprises an upper floating plate, a middle floating plate and a lower floating plate, the middle floating plate is hinged to the upper floating plate and the lower floating plate, the upper floating plate and the lower floating plate are connected through a spring, and the spring penetrates through the middle fl