The sum-difference beamforming network is a critical component of monopulse millimeter-wave phased array antennas. As a vital part of the feed system for monopulse phased array antennas, its amplitude and phase consistency directly determines key performance metrics such as the null depth of the difference beam, beam pointing accuracy, and sidelobe suppression. This paper proposes and designs a K-band sum-difference beamforming network composed of a broadband sum-difference device and a power divider network. This broadband sum-difference beamforming network demonstrates excellent amplitude and phase consistency in the K-band. The network is fabricated using a "sandwich" structure, where the entire stripline-based beamforming network is automatically press-bonded with upper and lower structural cavities after processing. Compared with traditional manual welding methods employing microstrip lines and structural cavities, this approach offers significantly improved manufacturing consistency, reduced production and assembly difficulty, higher production efficiency, and superior assembly yield. Through design, fabrication, and practical testing, the following performance metrics were achieved within the Fl ~ Fh frequency range: VSWR ≤ 2 for all ports；Port amplitude consistency (RMS): ≤ 0.21 dB；Port phase consistency (RMS): ≤ 2.8°。The test results demonstrate that this form of sum-difference beamforming network exhibits excellent performance and can be widely applied to engineering mass production tasks.