The invention provides a desulfurization solvent. The desulfurization solvent is prepared from the following components in percent by weight: 30-50% of N-methyldiethanolamine (MDEA), 0-20% of diisopropanolamine (DIPA) and 1-2% of protonation agent. The invention also provides a process for treating tail gas by applying the desulfurization solvent. The process is characterized in that the desulfurization solvent is used for absorbing sulfides in the sulfur-containing tail gas at the temperature of 30-50 DEG C, and the atmospheric pressure is maintained in the range of 100-600kPa in absorbing. The desulfurization solvent has higher selectivity for H2S in acidic gas, higher reaction activity, higher COS removing rate and lower energy consumption in regeneration reaction. The process provided by the invention has the advantages that under lower vapor consumption, the concentration of the H2S in the purified gas is reduced to below 20mg/m3, simultaneously the content of COS in the purified gas is removed to below 20mg/nm3, and the energy is saved by more than 50%, so that double objectives of standard emission, energy conservation and consumption reduction are realized and the optimal comprehensive benefit is obtained.