Transformation of phosphorus forms in the construction process of biological phosphate reduction system was discussed in treating saline and high-phosphorus pickled mustard tube wastewater to resolve problems encountered with present phosphorus removal technology. A phosphate reduction system under hypersaline condition (salinity, 7% , measured by NaCl) was successfully constructed, when the reactor was started with 3% salinity wastewater and the salinity was increased through two stages, at influent organic loading (COD) 0.45 kg x (m3 x d)(-1), phosphorus loading (PO4(3-)-P) 5.0 g x (m3 x d)(-1), DO 6 mg/L, 30 degrees C and without sludge discharge. Phosphorus balance and transformation of phosphorus forms in sludge was studied on 26 running cycles of this phosphorus reduction system. Results show that averagely 41.8 mg x L(-1) of external phosphorus gets lost per day, and that 155 mg of internal phosphorus in sludge gets lost through the path of phosphorus forms transformation and phosphate reduction, which accounted for 14.2% of the external phosphorus loss and 12.5% of the total phosphorus loss, respectively. The transformation path of phosphorus forms in sludge is Org-P --> NaOH85-P --> HCl-P --> NaOH-P --> BD-P --> H2 O-P.