The invention discloses a desulfurization and denitrification method of a desulfurization and denitrification system. The system comprises a tower body, wherein a flue gas inlet is formed in the lower part of the tower body; an oxidizing section is arranged at a lower position, corresponding to the flue gas inlet, in the tower body; a concentrating section, an absorbing section, a purifying and water washing section and a defogging section are sequentially arranged from bottom to top at an upper position, corresponding to the flue gas inlet, in the tower body. The invention also discloses a desulfurization and denitrification method of the desulfurization and denitrification system. The desulfurization and denitrification system provided by the invention is ingenious and reasonable in structural design, effectively solves the problem of high operation cost, and is stable in process; absorption, concentration and crystallization are carried out integrally, no any waste liquid is discharged, ammonia fugitive emission is limited, and the problems of ammonia escape and aerosol are thoroughly solved. The desulfurization and denitrification method provided by the invention is simple and easy in process steps, not only realizes the effects of desulfurization and denitrification, but also has the characteristics of high desulfurization and denitrification efficiency, energy conservation and environment protection, low cost and stable operation safety, and can make waste profitable.