A focus adjustable method of an optical image is implemented by a numerical analysis. An external light beam is allowed to pass through a lens module, thereby generating an optical image. The focus values of the optical image at four corners thereof are computed according to specified formulas to obtain a two-dimensional geometric tilt vector T(X,Y), an image diagonal spinor D and an image tetragonal average value M. According to the values T(X,Y), D and M, the tilt amount and the tilt direction of the lens module are adjusted, the optical quality of the lens module is discriminated and the focus value distribution of the lens module is determined.