In high power laser driver, frequency modulation to amplitude modulation (FM-to-AM) effect is a key parameter that could restrict fusion ignition, so qualitative investigation and evaluation are necessary. Based on acceptable bandwidth of the crystal, the physical mechanism of FM-to-AM effect in the process of frequency conversion is investigated from aspects of spectrum aberration and instantaneous frequency, respectively. Using instantaneous frequency, the correlation between phase modulation and intensity modulation is qualitatively analyzed, and the variation regularity of modulation frequency regarding the third harmonic generation (THG) intensity at the optimal phase matching angle is presented. With analytical derivation of modulation degree model, the modulation characteristics of output THG laser are quantitatively analyzed and the influences of input bandwidth and power density on THG intensity modulation are quantitatively studied as well. By numerical simulation, the reliability of the model is verified. All obtained results can give theoretical guide to suppress intensity modulation induced by FM-to-AM effects in THG process.