To improve the resolution and operating range of ladar, the transmission signal should be wideband phase-modulated signal or wideband linear frequency modulation (LFM) signal, for they have large time bandwidth product. It is hard to generate wideband LFM signal within short pulse width using the present laser frequency modulation technology. Therefore a method based on Mach-Zehnder modulation technique is studied to generate laser wideband LFM signal. Using one dual-parallel Mach-Zehnder modulator (DPMZM) can realize IQ quadrature modulation and generate laser LFM signal.Based on this, a method to generate laser-band frequency tripling LFM signal by using two DPMZMs is proposed. Theoretical analyses and simulation results show that the proposed method has good ability in harmonic suppression, and can greatly decrease the bandwidth of input modulation signal and can be realized with low-speed digital to analog conversion.