  Let $G$ be a simple connected graph with the vertex set $V(G)$ and
$d_{B}^2(u,v)$ be the biharmonic distance between two vertices $u$ and $v$ in
$G$. The biharmonic index $BH(G)$ of $G$ is defined as
$$BH(G)=\frac{1}{2}\sum\limits_{u\in V(G)}\sum\limits_{v\in
V(G)}d_{B}^2(u,v)=n\sum\limits_{i=2}^{n}\frac{1}{\lambda_i^2(G)},$$ where
$\lambda_i(G)$ is the $i$-th smallest eigenvalue of the Laplacian matrix of $G$
with $n$ vertices. In this paper, we provide the mathematical relationships
between the biharmonic index and some classic topological indices: the first
Zagreb index, the forgotten topological index and the Kirchhoff index. In
addition, the extremal value on the biharmonic index for trees and firefly
graphs of fixed order are given. Finally, some graph operations on the
biharmonic index are presented.
