A new method is proposed to solve the nonlinear problem in pretension design of cable-truss spaceborne reflector antenna. An optimization model, in which the cable initial length is treated as design variables, the reflector surface error as the objective and structures stress as constraints is established to seek the initial equilibrium state. In order to overcome space antenna import and relaxation shadow area appears cable problems, especially the introduction of strain constraints. To solve this highly nonlinear optimization model, cable strain and the displacement of reflector nodes are treated as linear function of cable length in local region. The optimization models are transferred into a series of quadratic programming, which can be easily solved. A simulation of a 10 m space antenna illustrated the efficiency of the method proposed.