The blood taken from a healthy human vein is irradiated by different wavelength lights at 502 nm, 530 nm and 632.8 nm. The fluorescent spectra are tested by the grating spectrograph. The result indicates that the fluorescent intensity of the blood induced by the light at 530 nm is the strongest. The fluorescence of the blood induced by the light at 502 nm is very weak. The emitting spectrum of the blood induced by the light of 632.8 nm appears in both side of 632.8 nm. This result shows that the processes of interaction between laser and blood may be different when wavelengths of the exciting lights are different. So the biological effects of the lights on the blood may be different.