The age of bloodstains is tightly related to the time elapsed since the crime was committed. The inference of the time that the crime was committed is of great significance to solve the case, and it was also a difficult problem in judicial authentication. Therefore, establishing a method of rapid determination of bleeding time is very necessary. Using a UV-visible spectrometer with a reflection accessory called ISR-240A and whiteboard as a reference standard, the reflection spectra of blood gauze, aluminum, glass and plastic were measured every hour under the condition of 16 °C and 70% humidity within 8 hours. Using SPSS to process the data, R541/R577 was figured out and linear fitting was completed. The same method was carried on on the gauze of blood at 24 °C. The results showed that, within 8 hours, spectral reflectance values of blood at 541 and 577 nm increased gradually with the passage of injury time. In addition to the R2 of the glass with blood is 0.769, the rest of the R2 were greater than 0.900. The values of F were greater than F0.05 (1, 6) = 5.59. Therefore, the linear regression model is significant meaningful. The method of using a UV-Visible spectrometer without doing any operations of the test samples is simple. Moreover, it does no harm to the further inspection in aspects of personal information, which is suitable for the judicial practice.