Abstract: The ionospheric heating experiments were carried out with the EISCAT heating and diagnosing facilities on September 13, 2010 at Tromsø, Norway. The long-lasting enhanced plasma lines were first observed by the ground-based VHF radar. Through the analysis of the plasma spectra, a cascade structure is discovered, inferring that the enhanced lines observed in the experiment may be caused by the parametric decay instability (PDI). In addition, an inversion method of electron density for the modified ionosphere is developed by using the cascade spectrum, which is verified by a quantitative comparison with the traditional grand unified incoherent scatter design and analysis package (GUISDAP) inversion that utilizes the reliable UHF radar data to hold an average deviation of~1% for the inversion results. This method is of significant importance to evaluating the parameters of strongly modified ionosphere and improving the understanding of the power absorption of radio waves and the electron acceleration by the Langmuir turbulence.