To accurately obtain discrete relaxation time spectra of asphalt binders, an algorithm termed the improved windowing interconversion method (IWIM) is presented in this paper. The IWIM allows discrete retardation time spectra to be converted into discrete relaxation time spectra, and vice versa. Compared with the windowing interconversion method (WIM), the IWIM affords a more exact distribution of time constants without generating negative spectrum lines. The IWIM was applied to the discrete retardation time spectrum of bending beam rheometer (BBR) test data for a conventional unmodified asphalt binder, successfully determining the discrete relaxation time spectrum. The result of the validation by means of a BBR test finite element simulation shows that the discrete relaxation time spectrum from the IWIM can well reproduce the stress relaxation properties of the asphalt binder.