Let $G=(V(G),E(G))$ be a simple connected graph with vertex set $V(G)$ and edge set $E(G)$. The atom-bond sum-connectivity (ABS) index was proposed recently and is defined as $ABS(G)=\sum _{uv\in E(G)}\sqrt{\frac{d_G(u)+d_G(v)-2}{d_G(u)+d_G(v)}}$, where $d_G(u)$ represents the degree of vertex $u\in V(G)$. A connected graph $G$ is called a unicyclic graph if $|V(G)|=|E(G)|$. In this paper, we determine the maximum ABS index of unicyclic graphs with given diameter. In addition, the corresponding extremal graphs are characterized.