Objective To evaluate the effect of dimethyl sulfoxide （DMSO） on micro-shear bond strength and the micro-morphology of bonding interface in the dry and wetted state of fluorosed dentin. Methods 120 freshly extracted non-carious fluorosed teeth were collected and grouped according to the Thylstrup-Fejerskov Index （TFI） for the severity of fluorosis: mild group （TFI = 1-3），moderate group （TFI = 4-5） and severe group （TFI = 6-9）. Each group was divided into four subgroups according to the different interventions: dry bonding group，DMSO dry bonding group，wet bonding group，and DMSO wet bonding group. After the bonding was completed，specimens of each group were subjected to microshear bond strength test to measure the bond strength. Specimens from each group were selected to observe the pretreated dentin surfaces under scanning electron microscope，and the resin-dentin bonding interfaces were observed under laser confocal microscope. Results The bonding strength of fluorosed dentin was affected by different bonding methods，and the results were statistically significant （F = 200.29，P < 0.05）. The bonding strength of wet bonding group was higher than that of dry bonding group，while DMSO pretreatment significantly increased the bonding strength of both dry and wet bonding groups，which was most obvious in wet bonding group. The bond strength decreased with increasing severity of fluorosis and the results were statistically significant （F = 89.87，P < 0.05）. The dry bonding group and the DMSO dry bonding group both showed intra-tubular cracks in dentin and closely arranged collagen fibers，while the wet bonding group and the DMSO wet bonding group did not show intra-tubular cracks and the collagen fibers were fluffy and porous in a reticular pattern. Confocal observation showed that the resin protrusions were short and sparse in the dry bonding group but long and dense in the wet bonding group. DMSO pretreatment could make the hybrid layer and resin protrusions thicker and longer in both the dry and wet bonding groups. Conclusion DMSO can effectively improve the bond strength of fluorosed dentin in dry and wet condition，but the bond strength under wet condition was higher than dry condition. The bond strength decreased with the severity of dental fluorosis.