The decrease of overlap between the reference spectrum and measurement spectrum (ReS and MeS) is found to limit the strain measurable range of the optical frequency domain reflectometer (OFDR) and signal-to-noise ratio (SNR) of cross-correlation results. In this Letter, the local similar characteristics of Rayleigh scattering (RS) fingerprint spectrum are discovered and, thus, an improved OFDR based on local similarity of RS spectrum is proposed to ensure the feasibility of the OFDR during measuring a large distribution strain with a high spatial resolution. The normalized length and location of the local spectrum are optimized to obtain a high spectral similarity between the ReS and MeS and achieve a robust suppression on cross-correlation fake peaks and multi-peaks. By this method, four times enhancement of similarity is experimentally achieved, compared to conventional OFDR methods when the measured distribution strain varies from 0 to 3000 μϵ with a 3 mm long fiber gauge. The experimental results verify that it effectively avoids influences of fake peaks and multi-peaks. The advantages of this method are a large strain measurable range, robust performance, a high SNR, and applicability with current OFDR systems.