A low-profile CTS flat-plate array antenna includes a radiating layer, a mode switching layer and a feed network layer which are sequentially arrayed from top to bottom. The mode switching layer comprises a first metal plate and a mode switching cavity array arranged on an upper surface of the first metal plate and comprising 22n mode switching cavities arrayed in 2n rows and 2n columns, wherein n is an integer greater than or equal to 1. Each mode switching cavity includes a first rectangular cavity, a second rectangular cavity, a third rectangular cavity, a fourth rectangular cavity and a fifth rectangular cavity which are sequentially connected from left to right. The 2n mode switching cavities located in each row are sequentially connected end to end.