A novel phosphine-based tetragonal Ag₄L₂ cage has been assembled from a tetraphenylethylene (TPE)-based tetraphosphine and Ag⁺ ions. The cage structure is maintained both in solutions and in the solid state. Ag₄L₂ emits strong fluorescence not only in dilute solutions but also in aggregated states. The turn-on fluorescence of TPE-P4 in dilute solutions is attributed to the formation of Ag₄L₂. Ag₄L₂ shows potential as a fluorescent sensor towards anions and olefin compounds, which arises from binding of the guest anions/olefin compounds by its labile silver centres.