Abstract: Soluble reactive phosphate (SRP) and dissolved total phosphorus (DTP) of colloidal phase (molecular weight>1 kDa, particle size<0.45 μm), true solution phase (molecular weight<1 kDa) and dissolved phase (particle size<0.45 μm) in water of Jiulongjiang Estuary were separated and extracted by the cross flow ultrafiltration technique. The environmental chemical behaviors of various forms of different phase phosphorus are preliminarily discussed. As result, the SRP and DTP system blanks of the crossflow ultrafiltration process were ND-0.001 mg/dm³ and ND-0.005 mg/dm³, respectively, which were no higher than the detection limit of the method. The sample quality recovery R was 92.3%-100.0%, satisfying the technical quality requirements of 80.0%-120.0%. The proportions of SRP, DOP and DTP in the dissolved phase, dominated by the real solution phase, were in ranges from 65.3% to 92.3%. The contents of SRP, DOP and DTP in the colloidal phase were (0.13-4.19) μg/dm³, (3.25-19.20) μg/dm³ and (3.39-23.40) μg/dm³, respectively, accounting for 1.2%-17.0% of the dissolved phase. The forms of phosphorus in the true solution phases and dissolved phases were mainly from the terrestrial inputs.