The optical effect of laser on protecting wheat from ultraviolet-B (UV-B) damage was tested. A patented instrument, coherent-to-incoherent optical converter, was employed to transform semiconductor laser into incoherent red light. The wavelength, power and spot diameter of incoherent red light are the same as that of semiconductor laser. A semiconductor laser and incoherent red light with wavelength of 650 nm and power density of 3.97 mW/mm2 directly irradiated the embryo of wheat seed for 3 min respectively, and when the seedlings were 12 days old they were irradiated by 10.08 kJ/m2 UV-B radiation for 12 h in darkness. Changes in the concentration of malondialdehyde (MDA), UV-B absorbance compounds, soluble protein, chla, chlb, the activities of peroxidase (POD), catalase (CAT), superoxide dismutase (SOD), and the growth parameters of seedlings (root length, root dry weight) were measured to test the optical effect of laser. The results showed that semiconductor laser pretreatment could enhance the SOD, POD and CAT activity, UV-B absorbance compounds, soluble protein, chla and chlb concentration, and root length, while incoherent red light pretreatment could not. When the cells of plant were irradiated by UV-B, concentration of cyclobutane pyrimidine dimers (CPDs) in wheats leaves was detected by enzyme-linked immunosorbent assay (ELISA) method, incoherent red light treatment could not eliminate active oxygen and prevent lipid peroxidation in wheat. The results also demonstrate that the plant DNA was injured by UV-B radiation and the semiconductor laser irradiance has the capability to protect plant from UV-B-induced DNA damage, while the incoherent red light could not. It is suggested that the potential mechanism is not the optical effect of laser treatment.