The seasonal dynamics of soil P fractions was determined in the Sanjiang plain marshes. The soil samples were taken from a depression marsh once or twice every month during the growth season and soil P fractions were determined by the modified Hedley fraction method in 2005. The study showed that P was mainly organically bound in the Sanjiang Plain wetlands. NaOH extractable organic P (NaOH-P_o) and inorganic P (NaOH-P_i) was the predominant form in total organic P (TP_o) and total inorganic P (TP_i); respectively. The content of Resin-P was highest at end of August and lowest at the early days of July, seasonal dynamic of NaHCO₃-P_i and NaOH-P_i were similar to Resin-P. Dil.HCl-P_i and Conc.HCl-P_i had their highest and lowest value at the beginning of September and the initial stage of May. The variation coefficient of Resin-P and Conc.HCl-P_i were higher than other P_i fractions, the level of them decreased from the beginning of growth season to the end of growth season, while the other P_i fractions increased in some degree. For organic phosphorus (P_o), the variation coefficient of NaOH-P_o was highest in the P_o fractions, and NaOH-P_o content reduced in the end of the growth season, NaHCO₃-P_o, Conc.HCl-P_o and Residual-P had no obvious changes during this time. The concentration of TP and TP_o had significant correlation and the concentration of TP was similar to TP_o. The dynamic of P fractions, especially inorganic P forms are intensively affected by the plants, the change of their value go with the absorbing intensity of plants, especially Resin-P. The transformation of phosphorus is directly promoted and also indirectly improved by environmental factors (e.g. water and temperature) with their effect on the biomass or activity of microorganism. The soil water content plays an important role in sorption and desorption of NaHCO₃-P_i, NaOH-P_i from soil. The biomass or activity of microorganism usually influences mineralization of organic P. The P_i which is released from mineralization of P_o is usually first absorbed by mineral oxide and then transform into other more active form for utilizing of plants or microbe.