We develop a new fluorescence method for the sensitive detection of protein tyrosine phosphatase 1B (PTP1B) based on dephosphorylation-directed tricyclic DNA amplification cascades. This method exhibits good specificity and high sensitivity with a detection limit of 0.24 pM. Moreover, it can be applied for kinetics analysis, inhibitor screening, and the accurate detection of PTP1B in a variety of cancer cells.