Diethanolamine (DEA) has been extensively utilized as an alkaline buffer in current assays of alkaline phosphatase (ALP) activity in the past decades. While playing the role of a buffer, the chemical reactivity of DEA has been widely ignored in such assays. Herein, we report an interesting chromogenic interaction between DEA and 4-aminophenol (AP) in the presence of H2O for the first time, which inspires us to develop a novel DEA-participated ALP activity assay by using 4-aminophenyl phosphate (APP) as a substrate. This APP/DEA-based colorimetric approach has been proved to be comparable and even superior to the conventional p-nitrophenyl phosphate (pNPP)-based one, especially in the low ALP activity region, due to its higher sensitivity. The clear response mechanism and excellent sensing performance ensure that it can be further applied to determining ALP activity in real biological samples, screening potential ALP inhibitors in vitro, establishing ALP-enabled ELISA, and even fluorophore-assisted fluorescent ALP activity assay. It is demonstrated that this strategy not only possesses a good feasibility, but also exhibits a promising outlook for a series of ALP-related and -extended detections.