Abstract: The theory of triple-axis neutron spectrometer (TAS) was described briefly. With MCSTAS code based on Monte-Carlo method, the resolution function was calculated at elastic scattering. The energy resolution, the scan of vanadium sample scattering experiment and the resolution function shape of different transferred energy were simulated. The relation between the choice of measuring position in real experiment and the shape of the resolution ellipse was analysed. The result shows that simulation calculation can calculate the shape of resolution function rapidly.