Abstract: Objective Multipath must be eliminated as an important error source when improving relative positio-ning accuracy of GPS short baseline in practical deformation monitoring engineering such as for dams、high-rise buildings and big bridges.In this study,a new sideral algorithm based on a double residualin the GPS observation domain is proposed to mitigate mutipath errors.We calculated residuals for alldouble observations after resolving ambiguity while denoising those residuals for the first day.Theseresults were taken as a multipath model to remove the mutipath errors from the double difference ob-servation equations of the subsequent day by taking advantage of sidereal repeatability of every satel-lite.Clean double observations without multipath errors could be extracted,and a better baseline co-ordinate sequence was consequently calculated.The results show that this new method not only doeswell in the vertical precision but also reduces the root mean squre(rms)in horizontal coordinate preci-sion by 23%,as compared with traditional sidereal filter.