An electrochemical sensing device with a plasticized polyvinyl chloride (PVC) membrane electrode based on a porphyrin derivative of p-amino tetraphenyl porphyrin (p-NH2TPP) for mercury ion (Hg2+) has been fabricated. The p-NH2TPP based PVC membrane electrode exhibited a nice linear potential performance for responding to Hg2+ in the range of 1.0×10-8 ~ 1.0×10-3 mol/L with a slope of 27.54 mV/-pC (25°C) in citrate buffer solution (pH=2.5). The detection limit was evaluated to be 7.5×10-9 mol/L. No obvious interferences were observed from Na+, K+, Ba2+, Mg2+, Ca2+, and Cr3+. The electrode possessed good stability and reproducibility. Comparing with atomic absorption spectroscopy (AAS), the electrode can be well applied to the determination of trace amount of Hg2+ in environmental waste water samples with a recovery rate of 94.83 ~ 104.78%. The sensing mechanism of the porphyrin derivative carrier for recognition of Hg2+ has also been expounded by the formation of nice complexation effect between the lone pair electrons provided by four nitrogen atoms of the porphyrin molecule and unoccupied orbitals provided by the mercury ion.