The purpose of this study is to develop an efficientmethod for estimating the first excursion probability in non-stationary response process of an elastic multi-degrees-of-freedom (MDOF) system. A simplified definition for the envelope process of the MDOF system together with the half-period sampling approach is introduced and enables to derive the multivariate probability density function (PDF) of the envelope process. The integral of the PDF is carried out with the Monte Carlo integration method and gives the first excursion probability. The effectiveness of the presentedmethod is investigated through some numerical examples.