Frequency modulation to amplitude modulation (FM-to-AM) conversion is an important issue that can prevent fusion ignition with high power lasers such as the Laser MegaJoule (LMJ). On LMJ, most of the FM-to-AM conversion is expected in the so-called frequency conversion and focusing system, which is a nonlinear system. However, we propose linear transfer functions to compensate the effect of frequency conversion on FM-to-AM conversion. We show that most of AM distortion can be reduced by practical systems: for beam intensity up to $3\mathrm{GW}/{\mathrm{cm}}^2$ , the FM-to-AM conversion level can be divided by at least 2, and we almost cancel intensity modulation for intensities below $1\mathrm{GW}/{\mathrm{cm}}^2$ .