Abstract: The pre-tightening process of the stud for reactor pressure vessels with three steps used for stretcher which are orthogonal has been studied by software ANSYS, and the optimization analysis have been carried out. The numerical results showed as follows: The studs which were loaded symmetrically at the first step, at interval at the second step and sequentially at the last step have small non-uniformity and dispersion. The non-uniformity and dispersion can be decreased obviously when the studs were with variable loads at the last step, thus to strengthen the sealing of the reactor pressure vessel.