The present disclosure relates to a method for forming a semiconductor device with a conductive cap layer over a conductive plug. The method includes forming a first word line and a second word line over a semiconductor substrate, and forming a dielectric layer covering the first word line and the second word line. The method also includes forming a conductive plug between the first word line and the second word line, wherein the conductive plug is surrounded by the dielectric layer. The method further includes removing a portion of the dielectric layer to partially expose a sidewall surface of the conductive plug, and forming a conductive cap layer covering a top surface and the sidewall surface of the conductive plug. In addition, the method includes forming a bit line over the conductive plug, wherein the bit line is electrically connected to the conductive plug through the conductive cap layer.