This paper describes the intensive geomagnetic storms with the index Dst < (-200 - -300 nT) observed during the period of two last cycles of solar activity. The analysis of variations of ionospheric parameters is based on the use of the measurement data from a network of ionospheric stations located at different latitudes in the longitudinal sector from 60 to 150°E. The results of numerical simulation of ionospheric parameters during the geomagnetic storm in April 2000 are also presented, which show a good agreement of calculations and measurements of critical frequencies in the F2 layer. Long-term negative ionospheric disturbances, observed during geomagnetic storms, may be produced by the change of thermosphere composition.