A method of focus and a focusing apparatus and a detecting module are provided. The detecting module includes an ellipse curved-surface reflection device and a light detector. The ellipse curved-surface reflection device has a beam gate, a first focus and a second focus. A light beam is focused by a light focusing device, and is projected on a surface of an object to be detected through the beam gate. The ellipse curved-surface reflection device reflects the light beams reflected or scattered by the object. The light detector is disposed on the second focus for receiving the light beam reflected by the ellipse curved-surface reflection device to generate a detecting result, by which a distance between the light focusing device and the surface of the object to be detected is adjusted, so that the light beam is correctly focused on the surface of the object.