Abstract: The principle of realtime modal reconstruction algorithms used for adaptive optics (AO) systems was introduced. A novel sensor-eigen modal reconstruction algorithm was proposednd analyzed. The sensor-eigen functions were deduced by orthogonalizing the correlation matrix between Hartmann-Shack wave-front sensor and deformable mirror. Compared to the normal direct-gradient reconstruction algorithm, the effect of measurement noise to AO system decreases more easily, the real-time system is more stable and the performances of the whole AO system improves through the modal algorithms. This novel algorithm was applied to a 61-element AO system and the theoretical results were verified by the experimental ones.