Let $\mathcal{G}_{n, β^*}$ $(\mathcal{G}^*_{n,β^*})$ be the set of all (connected) graphs of order $n$ with fractional matching number $β^*$. In this paper, the graphs with maximal spectral radius in $\mathcal{G}_{n,β^*}$ and $\mathcal{G}^*_{n,β^*}$ are characterized, respectively. Moreover, a lower bound for the spectral radius in graphs with order $n$ to guarantee the existence of a perfect fractional matching is also given, which generalizes the main result of O [Suil O, Spectral radius and matchings in graphs, Linear Algebra and its Applications, 2020].