This paper covers the concept of blazed holographic grating and its optimum design method by Fourier synthesis. Theoretical formulae have been derived from the basis of scalar electro-magnetic theory. The numerical caloulating results have been analysed in detail and skills for calculating and choosing design parameters have been discussed by introducing two parameters, i. e. ratio of profile depth R and phase shift, in relation to grating diffraction efficiency. It is shown that two optimum values must be chosen from a series of values of R and φ in order to obtain the optimum diffraction efficiency of blazed holographic grating for certain diffraction order and to control the two parameters correctly in manufacturing praotioe.