Abstract: A method to measure the horizontal scale of the ionospheric irregularity using a single ionosonde is presented in this paper. Through the establishment of ionospheric irregularity vertical reflection model, the whole ionospheric irregularity can be divided into several small reflection sources. Each of these sources reflects wave transmitted to them back to the ionosonde station individually. Getting the phase front of each reflected waves from the Doppler spectrum of the echo. These phases can give the direction of arrival(DOA) of these reflected waves by phase interferometer theory . We can get the distribution of these sources by combining the DOA of these reflected waves with the height of the ionospheric irregularity measured by the ionosonde. The horizontal scale of the whole ionospheric irregularity can then be decided. Some results of observation obtained by the method derived in this paper are discussed also.