The present application relates to a method for connecting plug parts of an electrical plug-in connector, in which on a first plug part a hollow cylindrical retaining sleeve is disposed movably in axial direction relative to the first plug part, wherein the retaining sleeve is retained in a first position biased by a bias spring and the axial relative movement is effected against the spring force, and upon connecting the plug parts, a spring element disposed on the second plug part is pressed radially outwards in a connecting intermediate state, upon further axially pushing together the plug parts, the retaining sleeve is displaced axially backwards from the first position by the spring element, and upon further pushing together the plug parts, the spring element snaps radially inwards into a recess and the backward displacement of the retaining sleeve by the spring element is terminated and the retaining sleeve is automatically again brought into the first position by the bias spring, wherein the spring element is introduced into a first clearance between an outer side of the first plug part and an inner side of the retaining sleeve before contacting the retaining sleeve for the axial displacement thereof effected by the spring element. The present application also relates to an electrical plug-in connector.