The invention discloses an ash blocking prevention method for an air preheater. According to the method, an ash blocking prevention sub-bin is constructed at the cold end of a secondary air sub-bin and is adjacent to a primary air sub-bin; three streams of air sources, namely, intermediate hot primary air leakage air, hot primary air and cold secondary air are connected in parallel into the ash blocking prevention sub-bin, and the airflow size of each air source is controllable; different gases, namely, the intermediate hot primary air leakage air, the hot primary air and the cold secondary air can be introduced into the ash blocking prevention sub-bin according to whether the ash blocking tendency is obvious or not, and anti-blocking and energy saving for operation of the air preheater are considered. The method has the beneficial effects of being low in cost, high in operation reliability, better in low-temperature corrosion resistance and ash blocking prevention capability and beneficial to improving the operation safety and the economical efficiency of a boiler system.