In this paper, we prove the irreducibility of global descents for even unitary groups. More generally, through Fourier-Jacobi coefficients of automorphic forms, we give a bijection between a certain set of irreducible cuspidal automorphic representations of U ( n , n ) ( A ) \mathrm {U}(n,n)(\mathbb {A}) and a certain set of irreducible square-integrable automorphic representations of U ( 2 n , 2 n ) ( A ) \mathrm {U}(2n, 2n)(\mathbb {A}) . We also give three applications of the irreducibility of global descents. As a global application, we prove a rigidity theorem for irreducible generic cuspidal automorphic representations of U ( n , n ) \mathrm {U}(n,n) . Moreover, as a local application, we prove the irreducibility of explicit local descents for a couple of supercuspidal representations and a local converse theorem for generic representations in the case of U ( n , n ) \mathrm {U}(n,n) .