The structure preserving decoupling method for quadratic system is proposed based on similar transformation in this paper. Firstly, the problem to find two decoupling transformation is converted to the problem to find one similar transformation. Secondly, the coefficient matrices of decoupled system are identified according to the spectral information of original system. At last, based on theoretical derivation the similar transformations are given by solving linear singular equation numerically. The numerical experiments show feasibility of the method, and this paper shows a new point for quadratic system decoupling researches.