Abstract: Correlations of the Dst index with various interplanetary parameters are studied by using the solar wind and interplanetary magnetic field data from the ACE spacecraft. Among all parameters investigated, E_y, the down-dusk component of the interplanetary electric field is found to be best correlated with the Dst index. During strong and moderate geomagnetic storms, it is seen that 2 to 3 hours before the storm main phase starts, E_y suddenly begins to increase and that the increase of E_y, is almost "synchronous" with the decrease of the Dst, with a time a head about 2 to 3 hours. Such an "abruptly changing characteristic curve" of E_y is much easier to be identified than those corresponding curves of V、V²B_z、VB₂ and ε. Weak disturbances in the magnetospheric background field, fluctuations of both the IMF Bz and the interplanetary E_y all strongly influence the development of storms, causing multiple-step main phase decrease and enhancement of the storm intensity. The physical mechanism responsible for the close correlation of Dst with the interplanetary electric field is discussed. A viewpoint that the present study of multiple-step main phase storms needs to be improved is proposed.