Research conclusions: In the high altitude control surveying of the railway construction , distortion of reductions of elevation is mainly in the length deformation. So , it is the best choice to select the level surface with compensation used as plane of projection,and build an independent coordinates system based on the orthomorphic projection 3°bands. The method is simple conversion, furthermore, the result of calculated coordinates is very close to the coordinates in the original coordinate system of national unity. It is helpful for the link between inside and outside of the surveying area. If the line length has a wide span from east to west, and when the center of the surveying area is far from the central meridian, we should consider selecting” any local projective stripes" or other methods.