We propose a novel injection-locked OptoElectronic Oscillator (OEO) with ultralow phase noise based on two cascaded phase modulators, which is further applied to construct a frequency synthesizer. Thanks to the phase modulation, the output optical spectrum expands and the optical power keeps constant while passing through the optical fiber, which dramatically reduces the intensity noise induced by the nonlinear effects of the optical fiber. A dual-output MZI together with a balanced optical detector realizes the phase modulation to intensity modulation conversion and improves the signal to noise ratio of the OEO. The output frequency of the proposed OEO is 9.9999914 GHz with its sidemode suppression ratio larger than 85 dB, and the phase noise reaches –153.1 dBc/Hz at 10 kHz frequency offset which is 38.7 dB lower than that of Keysight E8257D. Moreover, a broadband, high performance frequency synthesizer is established based on the proposed OEO. Combining the DDS and PLL technologies, the proposed frequency synthesizer can cover 5.9～12.9 GHz range. The phase noise is around –130 dBc/Hz@10 kHz, the spur suppression ratio is better than 65 dB, and the frequency hopping time is less than 1.48 μs.