A portable system of measuring 37Ar has been developed. Measurement of 37Ar is realized by the proportional counter and spectrum analysis method. Plateau length, plateau slope and background have been analyzed. Compared with traditional counting method, the present method has the advantages of visibility, longer plateau length and smaller plateau slope. background is reduced by an order of magnitude which decreases the detection limit. Working gas of 10% methane improves the peak shape. Greater concentration of methane is not necessary because it does not make the peak shape better but needs higher working voltage.