Optical frequency comb is a kind of multi-wavelength laser source with uniform and coherent frequency lines. Recently, the application of optical frequency comb in different fields has been expanding, such as spectroscopy, atomic clock, optical communication and microwave photonics. In the past decades, there are three main methods to generate optical frequency comb: firstly, optical frequency comb is generated by model locked lasers with a fixed repetition frequency. However, this method is limited by the model locked lasers, and it has small adjustable range and inflexibility. Secondly, optical frequency comb is generated on micro ring or cavity with great Kerr nonlinearity on chip, and the micro ring and cavity are easily affected by temperature in this method. Third, optical frequency comb is generated by electro-optic modulation using electro-optic effect. This method is generally used to generate optical frequency comb outside the chip due to the limitation of electro-optic modulator. Supercontinuum traditionally is generated by inputting MHz femtosecond pulse into a highly nonlinear waveguide to broaden its spectrum. Recently, lithium niobate materials stand out among many materials because of its strong linear electro-optic effect and high modulation bandwidth. With the integrated lithium niobate modulator, ultra-low optical loss, high modulation bandwidth and low RF drive voltage can be achieved simultaneously. Moreover, integrated lithium niobate has become an ideal platform in nonlinear optics because of its large second-order nonlinear index and high third-order nonlinearity. In this paper, we use the push-pull Mach Zehnder modulation and a cascade phase modulator integrated on the lithium niobate platform to generate optical frequency comb, and then let the signal go through a dispersion compensation module to compress the time domain waveform into a femtosecond pulse. At last, the signal is input into the lithium niobite waveguide which has great second-order nonlinear and third-order nonlinear to generate the second harmonic in the waveguide. Then the second harmonic interferes with the fundamental wave to generate a supercontinuum spectrum. In this paper, our mathematical model includes both second-order and third-order nonlinear, and the process of generating the second harmonic and supercontinuum spectrum is simulated in numerical simulation. Firstly, we use the cascade lithium niobite to generate a pulse with 180 fs pulse width and 126.4 pJ energy. Then this femtosecond pulse is input into the lithium niobite waveguide to generate the supercontinuum spectrum by the nonlinear effect of lithium niobite. We analyze the spectrum evolution under different energy of input pulse. In the simulation, we discuss the appearance of dispersion wave in the process of supercontinuum. Then we make a simulation on smaller modulation depth in order to get a wider pulse signal, and observe the effect of different input energies on supercontinuum broadening under this modulation depth. Comparing the energy required for the supercontinuum broadening of the optical spectrum with the modulation depth of 21π and 28π, it can be found that the wider the pulse width is, the narrower the 10 dB bandwidth of the spectrum is, the higher the energy required to generate the supercontinuum spectrum is. Moreover, when the modulation depth is 21π, the spectral width is about 75% of the modulation depth of 28π, and the pulse width is about 1.25 times of the modulation depth of 28π. However, after passing through the same section of waveguide, the supercontinuum spectrum also needs about 3 times of the energy when the modulation depth is 28π. Therefore, we believe that the pulse width, namely the width of the spectrum, has a greater impact on the supercontinuum broadening of the signal spectrum than the energy of the input pulse. This work lists the pulse width, spectrum width and input energy of input pulse when the supercontinuum spectrum is generated in different modulation depths. At last, we also observe the relationship between the pulse width and input energy in a figure, and conclude that the input energy will increase approximately square with the decreasing of pulse width.