On the basis of previous studies on crustal horizontal layered model in the Yutian area，and together with continuous GPS observation of the Yutian seismic station，we roughly estimate viscosity coefficient of each layers． Based on the viscoelastic medium horizontal layered model，the viscoelasticity coseismic Coulomb stress changes of MS7． 3 earthquakes in 2008 and 2014 in the Yutian area have been calculated separately． The frequency of“direct”aftershock has been calculated from the viscoelasticity coseismic Coulomb stress change，which directly related to the main shock Coulomb stress change in the study area． Further more，we propose a method to estimate the magnitude of the subsequent maximum earthquakes，by comparing the theoretic and actual earthquake frequency． The results show that the magnitude of the subsequent maximum earthquake caused by the coseismic Coulomb stress change of 2008 Yutian MS7． 3 earthquake lies between MS7． 2 to MS7． 5，much more larger than that of actual earthquake with MS6． 3，but similar to the 2014 Yutian MS7． 3 earthquake． Since two Yutian MS7． 3 earthquakes have the same magnitude，closer spatial distance，shorter time interval，similar stress action and strong tectonic correlation，they can be seen as a generalized doublet events，consisting with the sequence type characteristics in this region． The effects of lower limit of magnitude and b-value of G-R relationship on the results have been discussed preliminary，and it shows that the lower limit of magnitude has little effect，but b-value has a greater influence on the results．