A structure improvement for Christmas bulb socket, which comprises a plurality of lamp assemblies; the lower part of the bulb socket is furnished with a cross-shaped plug stub and two sets of symmetrical cross-shaped through holes; after one set of the two copper wires of the bulb is plugged in different set of through holes, two different kinds of bulb assemblies can be formed; therefore, after the bulb assembly is plugged in a correct socket, no incompatible bulb assembly can be used in a lamp string.