In order to decide the determination method of available phosphorus in fertilizer grade ammonium polyphosphate，different extraction and determination methods are studied. The results show that for fertilizer grade ammonium polyphosphate，the extraction method of EDTA extraction solution boiling for 15 min under alkaline condition has a better repeatability than the original method.Meanwhile，the new method can help reduce experimental time. On the other hand，compared to original determination method，ammonium polyphosphate requires longer boiling period under acidic condition. A laboratories ring comparison test was conducted according to the new determination method，which revealed that the repeatability standard deviation was 0.0859，and the reproducibility standard deviation was 0.2321 of this method，which proves that the method had wide applicablity and high precisim. It is a good general determination method and can be used as a standard in the chemical industry.