An electrical connector assembly includes several casings juxtaposed to and connected to one another along an assembling direction, and a positioning bottom plate disposed at a level below the casings. Each casing includes a top plate, a side wall extending downwardly from a lateral side along a mounting direction transverse to the assembling direction, and one fixing pin projecting downward from a bottom end of the side wall. The positioning bottom plate is formed with several rows of fixing channel. Each row of the fixing channels is aligned with the fixing pins of a respective casing. When the juxtaposed casings are brought to fix securely on a printed circuit board supporting the positioning bottom plate, the fixing pins of the casings respectively extend through the fixing channels in the positioning bottom plate such that the positioning bottom plate absorbs an accumulated tolerance caused due to a juxtaposed connection among the casings along the assembling direction.