A calibration method for strain based on microwave true time delay is proposed for distributed optical fiber strain sensors. When a segment of the sensing fiber is subjected to strain, causing a change in the fiber's time delay, the phase of the microwave signal transmitted through the sensing fiber will correspondingly change. Based on this principle, a microwave signal is confined to propagate within the sensing fiber using a microwave photonic link. The phase change of this microwave signal is measured using a vector network analyzer, and the strain applied to the sensing fiber is derived, thereby enabling the calibration of the strain coefficient for the distributed optical fiber strain sensor. This method directly measures the change in fiber time delay to determine the strain value and achieves a measurement time as low as ms level. Experimental results demonstrate that the calibration accuracy of this method is 0.18 (°)/(km·με).