The polarographic adsorptive wave of magnesium meso-tetra（4-trimethylammoniumphenyl） porphine （TTAPP） complex in the buffer solution of methylamine （pi12） have been studied. Ep=-1.5V （vs, SCE）. The detection limit is 1.7×10,-7, M. The peak height is linearly proportional to magnesium concentration over a range of 0.004 to 0.1μ9/ml. The method has been applied to the determination of trace amounts of magnesium in hair and chemical reagents. The composition of Mg-TTAPP complex has been studied by "polarographic mole-ratio method". The results show that Mg: TTAPP in complex is 1:1, the conditional stability constant of complex is β’=（1.3±0.1）×10,7,.