For optical freeform surfaces with steep slopes, the accuracy of overall surface shape reconstruction by modal method is limited and cannot meet the requirements, and the local surface features cannot be characterized accurately. A method based on Zernike polynomials and radial basis function to reconstruct freeform surface is proposed to improve the freeform surface reconstruction accuracy. The entire freeform surface is decomposed into many circular subdomains. The local surface patch is fitted by Zernike polynomials acting as base functions in each subdomain and the overall surface is formed by radial basis function. Numerical experiments are carried out with reconstruction accuracy analysis for five different surfaces. According to the experiments, freeform surface reconstruction accuracy is better than nanometer level. The adaptability and high accuracy of the proposed method are verified, which has great application prospect in the manufacturing and measuring of modern optical systems. At the same time, a few key issues have been discussed and analyzed in the freeform surface reconstruction.