It is not reliable to estimate the saturation laser intensity and the photon energy for the highest order harmonic using Ammosov-Delone-Kraimov(ADK) tunneling ionization theory in generation of high order harmonic when pulse duration of drive laser is shorter than picosecond. In this paper, new expressions for the saturation laser intensity and the photon energy for the highest order harmonic for an arbitrary laser pulse duration have been derived. Based on the expressions, the optimized parameters have been achieved to generate high order harmonic.