The microwave interferometry has recently emerged as a novel suitable noncontact technology to monitor vibrations and displacements of various engineering structures. This paper presents a method to evaluate the possibility of microwave interferometry for long-term periodic measurement of dynamic deflection of the long-span railway bridge under dynamic loads (such as a train passing over the bridge) without any auxiliary equipment. In order to eliminate the refraction influences of transmitted electromagnetic waves in different monitoring atmospheric environment, the Essen-Froome empirical formula is adopted to calculate the atmospheric refractive index to make atmospheric parameters correction (APC) to improve the measurement accuracy. First, an experiment was carried out and two corner-cube reflectors were fixed on tripods in a flat playground, continuous stationary monitoring was carried out by Image By Interferometric Survey (IBIS-S) between 8 A. M. and 19 P.M. to validate the APC method. The resulting RMS errors reduce to 0.05 and 0.06 mm from 0.17 and 0.19 mm after and before APC which highlight the reliability and accuracy of the APC for the application of microwave interferometry. Second, a long-span of Beijing-Tianjin intercity railway was monitored twice with an interval of about 3 months by the proposed methods. The results show that the monitored long-span railway bridge is in a stable condition and microwave interferometry maybe an alternative technique for the long-term periodic measurement of dynamic deflection of railway bridges.