In this paper, we characterize self-adjoint and normal composition operators on Poisson weighted sequence spaces $l^{2}(\lambda)$. However, the main purpose of this paper is to determine explicit conditions on inducing map under which a composition operator admits a best normal approximation. We extend results of Tripathi and Lal [Antinormal composition operators on $l^2$, Tamkang J. Math. 39 (2008), 347-352] to characterize antinormal composition operators on $l^{2}(\lambda)$.