只需将其包装为\mathit
:
\documentclass{article}
\usepackage{amsmath}
\begin{document}
\begin{equation*}
N\equiv R_1\alpha NP+R_2\beta MP+R_3\gamma MN(\bmod\;MNP)
\end{equation*}
\begin{equation*}
N\equiv R_1\alpha NP+R_2\beta MP+R_3\gamma MN(\bmod\;\mathit{MNP})
\end{equation*}
\end{document}