A projector focusing method includes acquiring a plane angle of a light beam and acquiring a first distance and a second distance between two sides of a light beam edge displayed on a projection plane and a time of flight device after the time of flight device emits the light beam to the projection plane, acquiring a plane equation of the projection plane according to the first distance and the second distance, acquiring an optical axis vector of a digital micro-mirror device (DMD) disposed inside the projector, designating target coordinates of the DMD, converting the target coordinates to the projection target coordinates on the projection plane according to the plane equation, acquiring an customized emitting vector according to the projection target coordinates and a lens position of the projector, and acquiring an ideal focal distances of the projector according to the customized emitting vector and the optical axis vector.