When a major earthquake occurs, how aftershocks distribute catch the attention of the public. Recently, the concept of Coulomb failure stress is used frequently to study the earthquake triggering of main shock. However, there are some pendent problems in the calculation and application of Coulomb stress. In conventional calculation of Coulomb stress, the shear stress change is approximated to be the changed value along the slip on the rupture of main shock. In fact, the stress orientations may change after the event. It affects the shear stress on ruptures. We improved the formula of Coulomb stress and made comparison. We applied the improved method to Wenchuan earthquake. According to the stress measurement in Longmenshan and the three rupture models, we calculated the static Coulomb stress of Wenchuan earthquake. The results show big differences between conventional and improved calculation around the fault. Our investigation reminds us to concern the uncertainty of the rupture models when we use Coulomb stress to predict the aftershocks.