The invention discloses a light source phase noise test method and a light source phase noise test device. The method comprises steps: firstly, a phase modulator is arranged on one way of delay fiber of a fiber interferometer, and modulation signals are loaded in the phase modulator; then, light source signals are transmitted to the fiber interferometer along the fiber, and interference signals generated by the fiber interferometer are modulated through the modulation signals; and finally, a phase generated carrier demodulation method or a working-point control method is used for demodulating the modulated interference signals, phase noise signals of the light source are acquired, and measurement on the light source phase noise is realized. through arranging the phase modulator on one way of delay fiber of the fiber interferometer, the fiber interferometer with a short-range difference is delicately used, phase modulation and interference signal acquisition are carried out synchronously, the phase noise is calculated according to an interfering modulation waveform through a demodulation algorithm, direct measurement on the light source phase noise is completed, a high-frequency detector and a high-frequency acquisition system are not needed, the system structure is greatly simplified, and the product price is reduced.