In order to determine the effects of nutrient inputs on changes of phosphorus forms and phytoplankton growth in large shallow lakes, an enrichment bioassay was conducted using surface lake water collected from the Meiliang Bay of Taihu Lake in spring. The concentration of different phosphorus forms, phytoplankton biomass (chlorophyll a) and alkaline phosphatase activity (APA) was analyzed after the addition of different concentrations of inorganic nutrients. The results showed that the phytoplankton biomass increased significantly with the addition of phosphorus (P), but with no primary effect from nitrogen (N), which suggested the phytoplankton growth was mainly limited by P. The maximum growth rate and the highest concentration of chlorophyll both occurred in the SRP 0.015 mg x L(-1) treatment. Nitrate addition could improve the bioavailability of phosphorus, accelerate the phosphorus cycling process and promote the growth of APA. There was an induction-repression mechanism resulting in a negative relationship between APA and orthophosphate concentration. The APA was obviously stimulated under PO4(3-) -P ≤ 0.025 mg x L(-1). This paper researches the transformation and cycling process of phosphorus in water and the induction-repression mechanism between the APA and orthophosphate concentration. The result can help to reveal the compensation path of nutrients in algae growth process and provide a theoretical basis for the further reveal of the mechanism of algae outbreaks.