Abstract: To explore the extreme span length of partially earth-anchored cable-stayed bridges more deeply, on the premise that ultimate tensile and compressive strength are equal for steel girder, that is, tension of steel girder at cross anchor and end of tower are equal, with cable-membrane theory of cable-stayed bridge to solving ultimate span length of partially earth-anchored and ratio of ultimate anchor-span ratio key dynamic responses were derived and the calculated outcomes were compared with solution attained from one finite element model. The results show that the ultimate span length of partially earth-anchored cable-stayed bridges based on axial force of the girder is 1.4 times as that of the conventional cable-stayed bridge, and the ultimate anchor-span ratio is about 0.293. In this paper, the key derived dynamic responses are consistent with that of the finite element model. The error is due to the fact that the cable arrangement system is usually fan-shaped rather than a radiation pattern in the real bridge structure. The presented theoretical equation can meet the requirements of the conceptual design, and it is suitable for analyzing the key dynamic responses and conducting mechanical analysis of partially earth-anchored cable-stayed bridges.