Acid phosphatase (ACP) can be sensitively, conveniently and efficiently detected via a new fluorescent "Turn-Off/Turn-On" system. Inexpensive (NaPO(3))(6) is carefully introduced into this system as a quencher of the aggregation-caused quenching (ACQ) probe. In our method, the limit of detection (LOD) is quite low for detecting ACP, which is an important biomarker and indication of several diseases.