The invention relates to a phase shifter of an electrically adjustable antenna, and belongs to the field of antennas of mobile communication base stations. The phase shifter comprises a rectangular cavity, wherein the rectangular cavity is formed by axially connecting a plurality of short cylinders; a connecting box is arranged between two adjacent short cylinders; two sides of each connecting boxare provided with sockets for inserting the short cylinders; one ends of each short cylinder, which is positioned in each connecting box, is provided with a flange; each flange is abutted against theinner wall of each connecting box; a fixing piece used for fixing each sliding short cylinder is arranged in each connecting box; a bidirectional lead screw is rotatably arranged between the short cylinders at two sides in each connecting box; two ends of each bidirectional lead screw are rotatably arranged on the side wall of the connecting box which is jointed with each flange in a penetratingway; each fixing piece is a sleeve which sleeves on each bidirectional lead screw in a threaded way; each sleeve is provided with a connecting rod; the other end of the connecting rod is connected with each flange; the length of the rectangular cavity can be adjusted according to the length required by the site, and the installation time of the phase shifter is shortened.