As a rising nano-scale imaging apparatus,photon sieve has been widely used in nano-lithography,large astronomical telescope,aviation vidicon et.al for its following merit:high resolution,small bulk,low weight and easy replication. To pursue high resolution,the diameter of pinholes of photon sieve should be very small,even less than the wavelength of incidence light,but scalar diffraction theory is no longer valid for this condition. So vector diffraction theory must be used. The diffraction model is built based on vector diffraction theory,then design and optimize the structure of photon sieves. To review the validity of this model,numerical simulation has been done. The results show that the focusing performance of photon sieves designed by the vector diffraction theory is good; In near field,the vector diffraction theory model satisfies the design requirement while the scalar diffraction theory is not valid.