We proposed an efficient phase stepping method for the use in fiber-optic ESPI. To improve phase-stepping accuracy and efficiency, a fiber-optic Michelson interferometer was phase-modulated by a ramp-driven fiber stretcher, resulting in 4$pi$ phase excursion in the PD interference signal. The zero-crossing points of the signal, which have consecutive $pi$ phase difference, were carefully detected and used to generate trigger signals for the CCD camera. From the experimental results by using this algorithm, $pi$/2 phase-stepping errors between the speckle patterns were measured to be less than 0.6 mrad with 100 Hz image capture speed. Also it has been shown that the error from the nonlinear phase modulation and environmental perturbations could be minimized without any feedback algorithm.