In this paper, a model for spatial nonlinear optimization programming to obtain the prestresses of cables in the cable-stayed bridge is established, in which the bending strain energy of the beams for the bridge and the cable tower is the objective function, and structural stresses and the cable forces are the constraint conditions. In order to determine the prestresses of the cables of the bridge, the first order optimization method is used to solve the program. This method is used to obtain set of optimal cable tensions of a thousand-meter scale cable-stayed bridge under permanent loads, and the result of analysis showed that the method is easy to apply and effective.