According to the qualities variation of fresh loaches in refrigeration with different gas composition, the best gas composition of modified atmosphere packaging (MAP) and shelf life of fresh loaches were determined. After slaughtered, loaches were packaged with different gas compositions:CO2(75%) +N2(25%), CO2(50%) +N2(50%), CO2(25%)+ N2(75%) and 100% air. The changes of sensory, microbial, physical and chemical indexes were measured during storage at 4 ℃. During the whole storage period, the sensory quality of high CO2 packaging groups (CO2(75%)+N2(25%) and CO2 (50%)+N2(50%)) were superior to those of other groups; after 10 days preservations, the total numbers of colonies of high CO2 packaging groups were 4.08 and 4.36 lg cfu/g, which were lower than the threshold value 6 lg cfu/g, while the other groups exceeded the threshold value; the higher CO2 concentration used in packaging, the lower pH value of loaches reached in the later storage stage; the variety of a*, b* and L* of high CO2 packaging groups was smaller than those of other groups, which indicated that the high CO2 packaging had better effect on protecting colors; from the texture profile analysis, high CO2 packaging groups had better effect on loach muscle structure protecting; the TBA values of MAP groups were far lower than those of the air packaging during storage; after preservation for 10 days, TVB-N values of high CO2 packaging groups were 6.61 mg/100 g and 9.72 mg/100 g, which were within the standard range, while the other groups exceeded the standard. In conclusion, modified atmosphere packaging could obviously extend the shelf life of fresh loaches; the best gas composition was CO2(50%)+N2(50%), with which the loaches could be preserved 10 days under 4 ℃ refrigeration condition.