The invention provides a waveguide slot array antenna including a first dielectric substrate, a first metal layer, a second dielectric substrate and a second metal layer which are stacked successively. A rectangular cavity is arranged in the first dielectric substrate, and opposite side walls of the rectangular cavity are metalized cavity walls. A plurality of metalized through holes are arranged in the second dielectric substrate and are distributed along four sides of a rectangle. A plurality of radiation slots for irradiating signals are arranged in the metal layer and are collinear and parallel to the long sides of the rectangle. The first metal layer, the second metal layer and the plurality of metalized through holes enclose a substrate waveguide. The plurality of radiation slots communicate with the rectangular cavity. The substrate waveguide irradiates signals via the plurality of radiation slots, and the two opposite metalized cavity walls are used to concentrate the outwardly-radiated signals. The needed height for the antenna is lower, and the design is simpler.