The physical model of amplitude modulation formation in frequency conversion process is established by theoretical derivation, amplitude modulation of tripling beam can be caused by both amplitude modulation and phase aberration of fundamental beam. The evolution process of amplitude modulation during the third harmonic generation(THG) is researched by numerical simulation, the results show that tripling amplitude modulation caused by fundamental amplitude modulation can be reduced by optimize doubling crystal detuned angle, while the influence of fundamental phase aberration can be depressed by increasing accepting bandwidth of frequency converter. Moreover, because there is a little difference in the optimized tripling detuned angle between optimal THG efficiency and minimal temporal modulation, both factors in an optimal design should be synthesised.