It is applicable to highway or light-weight railway for the suspension bridge, because suspension bridge is flexible and its stiffness is too small. Based on the existing design and research, the reasonable and functional stiffness index of long-span railway suspension bridge is presented. Then the factors which include side-span to mid-span ratio, sag to span ratio, depth to span ratio, width-span ratio are analyzed and compared with the factors of highway suspension bridge, and the suitable factors for railway suspension bridge are summarized. Finally the contribution of different structure system to the stiffness and their exiting problem are discussed. It could be a reference for designers and further research.