The invention relates to an SF6 gas density relay, in particular to a method for determining the positions of locating pieces in the SF6 gas density relay. The SF6 gas density relay, for which the method is applicable, comprises a regulating lever, inching switches, an end holder and locating pieces. The end holder is fixed on the end of a Bourdon tube, and is connected with one end of a temperature compensation sheet. The locating pieces are fixed in the encloser with the front ends thereof extend under the end holder connecting the Bourdon tube and the temperature compensation sheet. The invention is characterized in that the Bourdon tube moves downwards with the decrease of the actual pressure of the density relay until the actual pressure of the density relay is less than or equal to the pressure at the minimum temperature. When the actual pressure of the density relay is less than or equal to the pressure at the minimum temperature, the end holder at the end of the Bourdon tube touches the locating piece and is prevented from moving downwards. The rated operating pressure of the density relay ranges from 0.20 MPa to 0.41 MPa. By adopting the technical proposal, the density relay does not fail in low temperature environment, thus ensuring reliable working of the system.