405 nm LED was chosen as excitation source,Rose Bengal (RB) was used as photosensitizer,early oral tumor can be detected. Normal oral mucosa can not be dryed by Rose Bengal,oral precancerous lesions or tumors can be dryed by RB. It is known that characteristic fluorescence spectroscopy of lesions which were dryed ranged from 560 nm to 600 nm through spectral measurement,however normal oral cavity has special fluorescence peak around 480 nm. When precancerous lesions become malignant tumor,the special fluorescence spectroscopy was changed. Typical fluorescence peaks of porphyrin appeared at 630 nm and 690 nm. Combined with the technology of light induced fluorescence and the method of fluorescence ratio,oral precancerous lesions can be diagnosed,the sensitivity and specificity can arrive 95% and 92.5%. This is a non-invasive,quick and early diagnosis method. It increases survival rate of oral cancer patients remarkably.