\documentclass[cn]{elegantbook}
\makeatletter
\let\c@tcb@cnt@lemm\relax
\makeatother
\elegantnewtheorem{lemma}{引理}{thmstyle}{lem}[theorem]
\begin{document}
\begin{lemma}
  test
\end{lemma}
\begin{theorem}
  test
\end{theorem}
\begin{lemma}
  test
\end{lemma}
\begin{definition}
  test
\end{definition}
\begin{theorem}
  test
\end{theorem}
\end{document}