A exact diagonalization solution method for solving the one-dimensional Holstein model of DNA is discussed in this paper. We compared the results of this paper with the former results of coherent state variational method （CVM） to check the reliability and accuracy of CVM. It is clearly seen that the ground state energy calculated by exact diagonalization solution method is considerably smaller than that of the CVM and can reach the saturation with smaller site number. With the strict method we can not only obtain the ground state energy, but also the spectra of all excited states. Therefore there is a significant improvement in the studying of Holstein model than the former CVM .