System for controlling urea injection of selective catalyst reduction apparatus and method thereof

A system for controlling urea injection of a selective catalyst reduction apparatus, comprising:

• a catalyst apparatus that reduces NOx included in exhaust gas;
• a control portion that determines a necessary ammonia amount by applying first driving conditions to a predetermined NOx map and determines a final urea solution amount that is to be injected to the catalyst apparatus, based on the necessary ammonia amount;
• and
• an injector that injects the determined final urea solution amount to the catalyst apparatus,
• wherein the control portion controls the injector to inject the determined final urea solution, and
• the control portion determines the necessary urea solution amount by applying an ammonia conversion coefficient of the urea solution to the determined necessary ammonia amount, and determines the final urea solution amount by applying a total wall-wetted urea solution amount to the determined necessary urea solution amount.