In satellite optical communication transmitter with reflective telescope of two-mirror on axis, a large mount of the transmitted energy will be obscured by the secondary mirror. A novel method based on diffractive optical elements (DOE) is proposed for offsetting it. The beam centrally obscured is reshaped to annulus beam by a diffractive beam shaper and another phase corrector, as a result central obscuration is avoided. Numerical simulation is carried out with fixed obscuraction ratio and truncation ratio, and variation of them, respectively. Subsequent numerical simulation reveals that peak energy of far field is raised about forty percent under the condition of obscuration ratio is 15, and truncation ratio is 1.5. And numerical modelling of the latter are as follows: the larger obscuraction ratio, the larger transmission efficiency of whole system and peak energy of the far field, the narrower the width of main lobe of far field pattern, and the lower relative increment of peak energy; The larger truncation ratio, the larger relative increment of peak energy of beam in far field. This work can improve the emission efficiency of transmitter and energy density of receiver plane in satellite optical communication system.