A novel method of broadening spectrum with LiTaO3 crystal by switching microwave signal modulation is presented in this paper. The theory model of phase modulation is illustrated, with the according formulae in details. The output result of broading spectrum in a certain microwave power is computed and simulated by the formulae. Furthermore, from a practical microwave phase modulator system framed in this paper, the actual spectrum is measured corresponding to variant level of modulating microwave power. The spectrum calculated by theory agrees with the spectrum of real system in shape, except for the disaccord of the level due to the impedance mismatch in practical transmission. Using the integrated waveguide eletrooptic phase modulator made of LiTaO3, the input light at 1053 nm is modulated by a 4.3 GHz microwave frequency, whose switching signal is 1 kHz repetition frequency and square-wave of 4∶6 duty cycle. The 90 percent of input power is broadened to the fundamental frequency, first and second side frequency. The both side powers are nearly same.