A long wavelength emission fluorescent and colorimetric chemosensor with high selectivity for H₂PO₄⁻ ions was designed and synthesized according to the twisted intramolecular charge transfer (TICT) mechanism. The sensor bears a 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd²⁺ metal complex, which showed brilliant fluorescent and colorimetric response for H₂PO₄⁻ anions in aqueous solution. The detection limit of the sensor towards H₂PO₄⁻ is 2.8 × 10⁻⁶ M, and other anions, including F⁻, Cl⁻, Br⁻, I⁻, AcO⁻, HSO₄⁻, ClO₄⁻ and CN⁻, had nearly no influence on the probing behavior. The test strips based on the 2-pyridine-1H-imidazo[4,5-b]phenazine and Cd²⁺ metal complex (S₂–Cd) were fabricated, which could act as convenient and efficient H₂PO₄⁻ test kits.