Let \(\lambda _{\textrm{sym}^2 f} (n)\) be the nth coefficient of the symmetric square L-function attached to a holomorphic cusp form f for \(\textrm{SL}_2({{\mathbb {Z}}})\) . In this paper, we study the asymptotic behaviour of \(\begin{aligned} \sum _{n\leqslant x} \lambda _{\textrm{sym}^2 f} (n^m), \end{aligned}\) and establish asymptotic formulae for \(\begin{aligned} \sum _{n\leqslant x}|\lambda _{\textrm{sym}^2 f} (n^m)|. \end{aligned}\) Moreover, we prove an equidistribution result on the signs of \(\lambda _{\textrm{sym}^2 f} (p^m)\) . We are able to refine or extend previous results.