The invention discloses an online measuring device for the contact resistance of a rotary plug-in electrical connector. The electrical connector manual plug-pull and measurement method is slow, and has large error. The online measuring device comprises a heating unit, a socket clamping unit, a plug clamping unit, a planetary gear reducer, a solid shaft driving motor, and a mechanical control unit. The socket clamping unit is composed of a socket fixing plate, diamond polished rod support seats, compression springs, polished rods, a vertical bracket plate, self-lubricating linear sliding bearings, a plate rib, a plate rib base, an insulation plate, a cushion block, a moving bottom plate, a screw, a slide rail, a slider, a screw driving motor, a contact switch, and a pipe organ type insulation protective cover. The plug clamping unit is composed of a gear box left plate, a gear box right plate, a vertical support plate, plug clamps, hollow shafts, bearing retainer cups, a small gear, a big gear, an insulation ceramic pipe, bearing covers, and solid shafts. The influence of vibration and other factors during manual measurement of contact resistance is reduced. The contact resistance can be measured accurately and quickly.