The Doppler frequency shift of the radio echo reflected from the ionosphere is an important characteristic to measure ionospheric movements. Generally, for the Digisonde 256 digital ionosonde, a Doppler frequency shift with high resolution can be obtained in the drift mode. In this mode, the height profile of ionospheric disturbances cannot be acquired due to only a few sounding frequencies. In the routine ionogram mode, the profile of electron density is displayed, but the resolution of the recorded Doppler spectral lines is too low to detect precisely the ionospheric disturbances. This paper introduces a method to acquire a Doppler ionogram with high accuracy in the ionogram mode for Digisonde 256. In the condition of a narrowband ionospheric echo in the range bin, through converting the 16 channel data from the frequency domain to the time domain by Inverse Discrete Fourier Transform (IDFT) and estimating the Doppler frequency shift by a least squares method, a Doppler ionogram with high accuracy can be acquired. In order to validate the method, some experiments have been carried out with the Digisonde 256 at Wuhan Ionospheric Observatory. Based on an event of the Traveling Ionospheric Disturbance (TID), we describe how to use these derived Doppler ionograms to study the ionospheric disturbances. Those observation results show that the present method is very useful in the experimental investigations of ionospheric disturbances.