The changes in porcine myocardial metmyoglobin (pMetMb) and horse myocardial metmyoglobin (hMetMb) under ultraviolet irradiation are studied comparatively by synchronous fluorescence spectra. The ultraviolet irradiation is designed for the six gradients of 0, 5, 10, 20, 30, 60 min. With the irradiation of ultraviolet light, disintegration happens in the senior structure of MetMb. Synchronous fluorescence spectra can identify the influences of ultraviolet induction on tyrosine (Tyr) and tryptophan (Trp). The amino acid absorption value decreases for 0~5min, increases for 5~30 min, and decreases again for 30~60 min. It is identified that the ratio of Trp in the protein peptide chain is higher than the proportion of Tyr and two kinds of fluorescence spectra in pMetMb and hMetMb have similar changes. Synchronous fluorescence spectra further identify the heme-Fe3+ fluorescence spectra sign change in MetMb. The heme-Fe3+ fluorescence intensity is reduced when MetMb has been exposured for 5 min, and fluorescence intensity becomes higher for 10 min. After 20 min, the senior structure of heme-Fe3+ may be degraded or destroyed obviously, that may be fluorescence spectra variation characteristics in MetMb caused by the loss of biological activity.