The content of phosphine in maize could exceed the standard after maize being fumigated, which caused foods safety problem. Phosphides in maize in different storage period were tracking detected, the pretreatment methods on different level of heating time, acidity and temperature were tested by single factor experiment and orthogonal test with molybdenum blue colorimetric method, the test conditions were optimized: the water bath temperature 40 ℃, heating time 30 min, the ratio of sulfuric acid to water 1∶4. The changes of phosphide content in maize after fumigation showed that after three months the phosphide content in corn was safety.