Tradition Zernike modal wave front reconstruction method is improved in which a lot of surface integrations are avoided. Collimated thermally bloomed beam corrected by adaptive optics (AO) is numerically simulated. The results show that, because of model aliasing error, the Zernike reconstruction mode order is finite, which means that there exists a largest reconstruction mode order. When chosen reconstruction mode order is greater than the largest order, correction efficiency decreases quickly. And the largest order has no relation with thermal blooming strength. For a 69 elements AO system, which sub-aperture distribution is 8×8, the largest reconstruction mode order is 37. Compared with direct slope wave front reconstruction method, Zernike modal method can increase the adaptive optics system stability through choosing reconstruction mode when correction of thermal blooming distortes wave front.