Based on the mechanics characteristics of partially earth-anchored cable-stayed bridge, the criterion of reasonable finished dead state was presented. The effect factors considered include: the distribution of cable force, the inner force of girder and tower, the dimension of earth-anchor, and the displacement of tower top. Combined with methods to determine cable force of traditional self-anchored cable-stayed bridges in reasonable completed status, a two-stage method was used to find the cable force of these bridges which was divided into finding the initial cable force with rigid supported continuous beam method and then fix on the final optimized cable force in the reasonable completed status through adjusting the initial cable force in first stage with influence matrix method. And the adjustment process of cable force in secondary stage was proposed based on the mechanics characteristics of these bridges. Finally, a calculation was carried out for a partially earth-anchored cable-stayed bridge with 1218m main span and composite beam. The result shows that this method was logical, practical convenient and efficient. And the cable force in the reasonable completed status of bridges can be obtained faster according to the adjustment process of cable force in secondary stage.