A low-sidelobe plate array antenna includes a radiation layer and a feed layer. The radiation layer is superimposed on the feed layer and includes a first plate and a radiation array disposed on the first plate. The radiation array is formed by n2 radiation units which are distributed in 2(k-1) rows and 2(k-1) columns. Each radiation unit in the radiation layer is constituted by two first radiation assemblies and two second radiation assemblies. Each first radiation assembly comprises a first rectangular bar, a first rectangular cavity, a second rectangular cavity and a third rectangular cavity. The first rectangular cavity, the second rectangular cavity and the third rectangular cavity in the first radiation assembly are stacked in presence of an azimuth deviation to form a three-layer coupled structure, and the first rectangular bar located in the first rectangular cavity can better restrain cross polarization and reduce the sidelobe.