The invention discloses a desulfurization and denitrification method. A system adopted in the method comprises a tower, wherein a flue gas inlet is arranged at the lower part of the tower, an oxidation section corresponding to the lower position of the flue gas inlet is arranged in the tower, and a concentration section, an absorption section, a purification and washing section and a demisting section corresponding to the upper position of the flue gas inlet are sequentially arranged in the tower. The invention further discloses the desulfurization and denitrification method of the desulfurization and denitrification system. The desulfurization and denitrification system is ingenious and reasonable in structural design, the problem of the high operation cost is effectively solved, the technology is stable, absorption and concentration crystallization are operated as a whole, no waste liquor is emitted, ammonia escape emission is limited, and the problems of ammonia escape and aerosol are thoroughly solved. The desulfurization and denitrification method has simple technological steps, realizes desulfurization and denitrification effects, has the characteristics of high desulfurization and denitrification efficiency, energy saving, environment friendliness, low cost and operation safety and stability, can turn waste into wealth and is a key direction of environment-friendly desulfurization development in future.