Actual reflector antenna always has a deformation which has a serious influence on antenna performance. Establishing the equation of the distorted reflector surface is important for calculating antenna performance and precise control. A new best-fitting method based on least square method is submitted in this paper. This method depart from the ideal parabolic equation, and use the distance between the deformation point and the correspondent fitting point to establish the least square error condition equations, though which the six parameters can be got to determine the geometry position of the deformed parabolic accurately. Matrix operation can make the algorithm easy to realize in computer. Nan-shan 25m antennas finite element model is established in ANSYS program, and its result is analyzed compared with another method, confirmed the operability and accuracy of this algorithm.