此处相关的标点符号命令是\entrysetpunct
\documentclass{article}
\usepackage[backend=biber,subentry,style=chem-acs]{biblatex}
\renewcommand*{\entrysetpunct}{\addperiod\space}
\addbibresource{biblatex-examples.bib}
\begin{document}
A\autocite{set,stdmodel}
\printbibliography
\end{document}