Phosphorus form and availability of paddy soil may change after flooding, in order to know the change laws, selecting Huangnitu, a typical paddy soil, to cultivate at different flooding time in Taihu Lake Region, using fresh soil determination to study the effects of phosphorus transformation and availability at different flooding time. The results showed that flooding time mainly influenced the transformation of Fe-P, O-P, Ca10-P and Ca8-P. Compared with the previous flooding, Fe-P and Ca8-P increased but O-P and Ca10-P decreased. Fe-P increased significantly and then stabilized in 45 days, Ca8-P in 15 days, Fe-P and Ca8-P increased 194 and 9.6 mg/kg respectively (accounting for 93% and 73% before flooding). Compared with the previous flooding, Phosphorus availability and the amount of available phosphorus were to the highest level in 45 days, available phosphorus increased 32 mg/kg (accounting for 57% before flooding) at this time. Flooding time affects phosphorus form transformation/ availability and the amount of available phosphorus. The amount phosphorus form transformation and available phosphorus as well as phosphorus availability were to the highest in 45 days.