The invention discloses a method and a device for detecting the optical path difference between the beams of an optical fiber array point source generator. The detection method comprises the following steps: based on the optical path system of an all-fiber Mach-Zehnder interferometer, placing reflecting mirrors on the end faces of an optical fiber to make reference light and test light return to the optical fiber, and fixing a reference arm optical fiber on a piezoelectric ceramic micro displacement actuator, namely, PZT; changing the output voltage of the power supply of PZT to stretch the optical fiber and change the length of the optical fiber, and making the reference light and the test light return to a photoelectric detector and displayed by an oscilloscope; and calculating the fractional part of the phase difference through a phase shifting method, calculating the integer part of the phase difference through an excess fraction method, and finally, obtaining the optical path difference. The detection device, which is based on an all-fiber Mach-Zehnder interferometer, comprises a laser, first and second optical fiber couplers, an optical fiber array, first and second reflecting mirrors, a photoelectric detector, an oscilloscope, and a piezoelectric ceramic micro displacement actuator. The optical path difference of an optical fiber array point source generator can be detected accurately, rapidly and effectively.