The mathematic model of modal wavefront reconstruction using Hartmann Shack (H S) wavefront sensor is analysed. The effect of non orthogonalisty of the mode on the reconstrution precision is simulated, and it is indicated that the non orthogonality does not affect the use of modal algorithm. The conclusion is that the mode orthogonality is not a necessary condition in the course of wavefront measurement and correction using Hartmann Shack sensor, and the orthogonalizing algorithm cannot improve the measurement and correction precision in Hartmann Shack sensor and adaptive system.