High-order harmonic spectra are obtained by numerically solving the one-dimension time-dependent Schr?dinger equation with single-active-electron approximation. The influence of the initial phase of the excitation field on the single-atom high harmonic generation is analysized through Gabor analysis of the acceleration of the atomic dipole. The influence of the initial phase on the process of the high harmonic generation stronger for the shorter-duration pulses than for the longer case. A pulse train or a single pulse of subfemtosecond level could be obtained by high harmonic generation through the adjustment of the phase of the excitation laser field.