\usepackage{titlesec} %自定义多级标题格式的宏包

\titleformat{\section}[block]{\LARGE\bfseries}{Sec.\arabic{section}}{1em}{Hello: }[]

\titleformat{\subsection}[block]{\Large\itshape\mdseries}{\arabic{section}.\arabic{subsection}}{1em}{}[]

\titleformat{\subsubsection}[block]{\normalsize\bfseries}{\arabic{subsection}-\alph{subsubsection}}{1em}{}[]

\titleformat{\paragraph}[block]{\small\bfseries}{[\arabic{paragraph}]}{1em}{}[]