To estimate the thermal distortion of a large aperture SiC lightweight primary mirror exactly, the calibration and calculation methods for the SiC lightweight mirror were researched in different thermal modes. Temperature sensors stuck on the lightweight mirror were divided into three levels along an optic axial direction, and the temperatures at each level were fitted by quasi-Zernike polynomials, respectively. By taking a 4 m SiC lightweight mirror for an example,the Finite Element Method(FEM) built by MSC.Patran was used to calibrate and calculate the thermal distortion in eighteen kind temperature fields of the first ninth quasi-Zernike polynomials and the Max deformation, surface error PV and RMS values due to the lightweight mirror thermal distortion were obtained for different temperature patterns. Calculation results show that the deformation of lightweight mirror is max under the first quasi-Zernike polynomial temperature pattern, and the surface error RMS value is 278.3 nm. Using a least-square method, the aberration coefficients of surface errors are calculated by quasi-Zernike polynomial wavefront fitting program for each temperature field, and the main aberration caused by the thermal distortion is piston, tilt, defocus, coma and astigmatism.