Based on the time-dependent nonlinear coupled wave equations, the amplitude modulation (AM) effect of third-harmonic pulse in the frequency conversion device of broadband Ndglass is studied numerically. To suppress this AM effect of output third-harmonic generation pulse, an added narrowband Ndglass laser pulse is mixed with the broadband laser pulse in the sum-frequency process of tripling configuration. Numerical results show that the limited phase-matching bandband due to the group-velocity-mismatch effects are substantially alleviated with the assist of a narrowband pulse. As a result, for a required third-harmonic wave with 1 THz bandwidth, the defined relative modulation of pulse intensity is as large as 180% in the conventional baseline design, while it will be reduced to only 20% by using the proposed scheme.