In the study of the propagation of a high‐power microwave pulse, one of the main concerns is how to minimize the energy loss of the pulse before reaching the destination. In the very high‐power region, one has to prevent the cutoff reflection caused by the excessive ionization in the background air. A frequency autoconversion process that can lead to reflectionless propagation of powerful electromagnetic pulses in self‐generated plasmas is studied. The theory shows that under the proper condition the carrier frequency &ohgr; of the pulse shifts upward during the growth of local plasma frequency &ohgr;pe. Thus, the self‐generated plasma remains underdense to the pulse. A chamber experiment to demonstrate the frequency autoconversion during the pulse propagation through the self‐generated plasma is conducted. The detected frequency shift is compared with the theoretical result calculated by using the measured electron density distribution along the propagation path of the pulse. Good agreement is obtained.