An experimentally study on microwave frequency transfer over a 50 km long fiber was reported, which frequency instabilities reach 4.38×10^-15@1 s and 2.80×10^-18@65.5×10³ s. By using the well-known Doppler noise cancellation method, the fiber noise was measured by comparing the phase of the signal double-passed fiber and the reference one at local site, and compensate the fiber noise with a home-made fiber-based variable delay line, which includes a fiber stretcher driven by a piezo-transducer for fast action with 1 kHz bandwidth and a temperature controlled fiber spool with a delay adjustment range of 5 ns. Compared with electronic phase compensation technique, this approach is insensitive to microwave leakages which is difficult to avoid in such a case, consequently, enables a better instability for long terms. In addition, we synthesized the back and forward transferred signals into two different frequencies to avoid effect introduced by stray optical reflections, and employ dispersion-compensation fiber to reduce the penalty due to the chromatic dispersion.