  This paper studies classical weight modules over the $\imath$quantum group
$\mathbf{U}^{\imath}$ of type AI. We introduce the notion of based
$\mathbf{U}^{\imath}$-modules by generalizing the notion of based modules over
the quantum groups. We prove that each finite-dimensional irreducible classical
weight $\mathbf{U}^{\imath}$-module with integer highest weight is a based
$\mathbf{U}^{\imath}$-module. As a byproduct, a new combinatorial formula for
the branching rule from $\mathfrak{sl}_n$ to $\mathfrak{so}_n$ is obtained.
