An auto focusing lens module is provided, which includes a casing, a photo interrupter and a lens holder. The casing has a containing space. The photo interrupter is fixedly disposed on the casing and has a light-emitting surface and a light-sensing surface opposite to the light-emitting surface. The lens holder is disposed in the containing space and has a first obstacle and a second obstacle. The space between the first obstacle and the second obstacle keeps a calibrating interval. The lens holder is capable of moving to-and-fro along an axial direction to bring the first obstacle, the calibrating interval or the second obstacle to the space between the light-emitting surface and the light-sensing surface. The invention also provides a calibrating method suitable for the above-mentioned auto focusing lens module.