Research conclusions: To the steel—truss girder cable—stayed bridge，firstly，the length of the upper chord should be set as variables and the multiple constraint equations should be established to the influence vectors of the the length change of the upper chord on segment camber or other restriction conditions．Then，the solution circumscription should be given and the most closed ones should be solved to get the pre—camber that is the most closed to the theoretic results of the multiple constraint equations．Finally，the theoretic solutions should be rounded，merged and adjusted to obtain the rather ideal pre—camber．