We revisit the scheme of single-photon weak-coupling optomechanics using post-selection, proposed by Pepper, Ghobadi, Jeffrey, Simon and Bouwmeester[Phys.Rev.Lett.109, 023601 (2012)], by analyzing the exact solution of the dynamical evolution.Positive and negative amplification effects of the displacement of the mirrors position can be generated when the Kerr phase is considered.This effect occurs when the post-selected state of the photon is orthogonal to the initial state, which can not be explained by the usual weak measurement results.The amplification effect can be further modulated by a phase shifter, and the maximal displacement state can appear within a short evolution time.