AbstractWe investigated the variations of 74 microwave ZP structures observed by Chinese Solar Broadband Radio Spectrometer at 2.6–3.8 GHz in 9 solar flares, found that the ratio between the plasma density scale height L_N and the magnetic field scale height L_B in emission source displays a tendency of decrease during the flaring process, indicates that L_B increases faster than the L_N during solar flares. The detailed analysis of the step-wise decrease of L_N/L_B in three typical X-class flares reveals the magnetic field relaxation relative to the plasma density.