The invention discloses an installation computation method for closure rod pieces of steel truss girder cable-stayed bridges, and relates to the field of bridge building. The installation computationmethod includes carrying out a longitudinal displacement computation step, to be more specific, carrying out completed bridge computation and optimizing cable force of completed bridges according to specified line types of the completed bridges so as to obtain longitudinal displacement delta of movable supports at main towers; carrying out a closure-segment rod piece size computation step; carrying out a completed bridge computation step, and to be more specific, carrying out completed bridge computation according to the sizes of the latest closure-segment rod pieces; carrying out a judgment step. The installation computation method has the advantage that the bridge design cycle can be effectively shortened by the aid of the installation computation method.