  We study the modularity of the generating series of special cycles on unitary
Shimura varieties over CM-fields of degree $2d$ associated with a Hermitian
form in $n+1$ variables whose signature is $(n,1)$ at $e$ real places and
$(n+1,0)$ at the remaining $d-e$ real places for $1\leq e <d$. For $e=1$, Liu
proved the modularity and Xia showed the absolute convergence of the generating
series. On the other hand, Bruinier constructed regularized theta lifts on
orthogonal groups over totally real fields and proved the modularity of special
divisors on orthogonal Shimura varieties. By using Bruinier's result, we work
on the problem for $e=1$ and give an another proof of Liu's proof. For $e>1$,
we prove that the generating series of special cycles of codimension $er$ in
the Chow group is a Hermitian modular form of weight $n+1$ and genus $r$,
assuming the Beilinson-Bloch conjecture with respect to orthogonal Shimura
varieties. Our result is a generalization of $\textit{Kudla's modularity
conjecture}$, solved by Liu unconditionally when $e=1$.
