1
\documentclass{article}
\usepackage{amsthm}
\renewenvironment{proof}{{\bfseries Proof.}}{}
\renewcommand{\qedsymbol}{$\blacksquare$}
\newtheorem{theorem}{Theorem}[section]
\begin{document}
    \begin{theorem}
        blablabla
    \end{theorem}
    \begin{proof}
        abcdefghijklmnopqrstuvwxyz
    \end{proof}
\end{document}

I want to make the "proof title" is bold, so I give the code \renewenvironment{proof}{{\bfseries Proof.}}{}. But why after adding this code the square symbol in the end of proof is disappear? How to appearing the square symbol in the end of proof?

1

There is no symbol because you left the final argument in your \renewenvironment definition blank. You need to add \qedsymbol. You also need to load amssymb to get the $\blacksquare$ symbol.

\documentclass{article}
\usepackage{amssymb,amsthm}
\renewenvironment{proof}{{\bfseries Proof.}}{\qedsymbol}
\renewcommand{\qedsymbol}{$\blacksquare$}
\newtheorem{theorem}{Theorem}[section]
\begin{document}
    \begin{theorem}
        blablabla
    \end{theorem}
    \begin{proof}
        abcdefghijklmnopqrstuvwxyz
    \end{proof}
\end{document}

enter image description here

1

The amsmath package should not hardwire \itshape in the definition of amsthm. Maybe a future release will provide a way to choose the font.

In the meantime, it's not difficult to modify the environment by patching.

\documentclass{article}
\usepackage{amssymb}
\usepackage{amsthm}
\usepackage{xpatch}

\xpatchcmd{\proof}{\itshape}{\bfseries}{}{}
\renewcommand{\qedsymbol}{$\blacksquare$}

\newtheorem{theorem}{Theorem}[section]

\begin{document}

\begin{theorem}
blablabla
\end{theorem}
\begin{proof}
abcdefghijklmnopqrstuvwxyz
\end{proof}

\end{document}

enter image description here

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