When measuring glyoxal directly in the atmosphere by Long Path Differential Optical Absorption Spectroscopy (LP-DOAS), the analysis of glyoxal strongly suffers from the cross interference of other absorbers or structure (Xe lamp-structure, H2O, O4 and NO2 absorption). The retrieval method of glyoxal-the elimination of interfering absorption or structure was studied. The Xe lamp-structure will change nonlinearly resulted from the change of pressure broadening and Doppler Broadening with the change of of Xe lamp pressure or temperature. The different lamp reference spectra detected at different time were interpolated to eliminate lamp structure and the resulting residual structures caused by an incorrect elimination were a factor of three below the detection limit of glyoxal; high amount of H2O in the atmosphere cause the nonlinear absorption of H2O and the observed band shape to vary with the column density of H2O. Two H2O absorption spectra with higher and lower column density were interpolated to eliminate nonlinear H2O absorption and the resulting residual structures caused by an incorrect elimination were ten times below the detection limit of glyoxal. In addition, the strong absorption of interfering species NO2 and O4 were also accurately removed. Low detection limit (0.15 ppbv) and low systematic errors (~10 %) were achieved by the accurate elimination of interfering structures. In the end, glyoxal was routinely detected during the daytime on the outskirts of Guangzhou, where mixing ratios ranged from less than detection limit (0.15 ppbv) to 1.66 ppbv. The variation and range of glyoxal concentration detected agreed well with the results reported.