In this paper, the effect of phosphate on As phytoextraction was examined. The effect of phosphate on As dissolution, the As uptake of plants, the internal plant translocation, and phytotoxic effect were investigated. Lupine plants were grown on As-contaminated soil collected from an industrial site containing 670 mg/kg As and were treated with biammonium phosphate (BAP). Two different BAP application procedures were tested: single-dose and multiple split additions. BAP was found to be effective in increasing the water-soluble As concentration in the test soil. As the concentration of water-soluble As increased, the Lupine plants responded accordingly with an increased As uptake. The As content in the shoots and the translocation factor were the highest when BAP was added in multiple split additions. On the contrary, a single application caused the highest As content in the roots and consequently the lowest translocation factor. In addition, it was established that the single-application method significantly reduced the plant biomass by twofold, this reduction being an evident phytotoxic symptom. Measurements of the combined biomass production and As content values revealed that the highest As phytoextraction is obtained with BAP applied in multiple doses which is about 14-fold higher than in the control plants, whereas a single-dose BAP application increased the phytoextraction rate only 1.6 times. These results demonstrate that significant improvements in the current phosphate-assisted phytoextraction of As could be achieved.