The paragraphs in Chpaters and Sections have indent. But the first paragraph of the subsection does not have indent.
In the .cls file, it is set as
\setlength{\parindent}{0pt}
And what should I do if I don't want indent in all paragraphs?
Please check the attached figure. Many thanks!

\setlength{\parindent}{0pt}if you have set that some code you have not shown has set it back. probably you can add\usepackage{indentfirst}to indent the first paragraph after a section head – David Carlisle Dec 22 '23 at 23:59