Nitrification inhibitors (NIs) are widely applied in soil to improve the nitrogen utilization efficiency. Currently, commonly used nitrification inhibitors, like 3,4-dimethylpyrazole phosphate (DMPP), are not resistant to high temperature and do not have a phosphate-solubilizing ability. In this paper, a novel polymer nitrification inhibitor (PNI) with nitrification inhibition ability, phosphate-solubilizing ability, and good thermal stability is chemically synthesized from acrylic acid (AA) and 3,4-dimethylpyrazole (DMPZ). The PNI has a high glass transition temperature (Tg: ∼78 °C) and effective decomposition temperature (Td: ∼216 °C). The good thermal stability makes it suitable for high tower granulation processes. The PNI demonstrates an excellent nitrification inhibition performance when the dosage is 0.5 wt % of the urea fertilizer and presents good phosphate-solubilizing ability when the volume ratio of PNI to water is about 1:1000. The PNI can be applied to the field to realize nitrogen immobilization and phosphate-solubilizing at the same time or be used as the synergistic additives for fertilizers such as water-soluble fertilizers, liquid fertilizers, and compound fertilizers.