The invention discloses a combustion system improving safety and economical performance of an opposite-firing boiler. The combustion system comprises a primary fan, a secondary fan, an air pre-heater, a hearth, a plurality of direct-flow overfire-air nozzle, a plurality of first overfire-air nozzles, a plurality of second overfire-air nozzles and a plurality of cyclone burners. By adopting the combustion system, the problems of the opposite-firing boiler that fly ash combustibles are increased, a water-cooling wall is coking, high-temperature corrosion is caused, difference of oxygen amounts at two sides of a hearth outlet is large, the vapor temperature is difficult to control, and the like can be effectively solved.