In order to improve the efficiency of alignment for a three-mirror-anastigmatic (TMA) telescope with large aperture, the influence of three point mount induced (TPMI) deformation on its astigmatic field distribution in the aligning process is analyzed. Based on the vector aberration theory and aperture coordinate transformation, the astigmatic field distribution characters for the aligned and misaligned TMA telescope are analyzed with TPMI deformation on mirrors located at or away from the aperture stop. Finally, the surface error due to TPMI deformation is simulated by the optical design software CODE V using Z10 and Z11, the terms of fringe Zernike polynomial. The characteristic field distribution of astigmatism is verified by the real-ray-tracing method. The results indicate that the astigmatic field distributions are not affected when the TPMI deformation is on the primary mirror (aperture stop), and it shows different characteristic field distributions when the TPMI deformation is on the secondary or tertiary mirror (not aperture stop) for the aligned or misaligned telescope because of producing astigmatism with the field conjugated and field linear. The aligning state of TMA telescope can be qualitatively analyzed by its astigmatic field distribution at the final aligning stage, which is quite instructive for the alignment of TMA telescopes with large aperture.