# How to indent a particular equation

I set the global setting as \setlength\mathindent{0pt} ,but I want to indent a particular equation.
How to do so?

A comment up front: I will assume that you've set \setlength\mathindent{0pt}, not \setlength\mathindent[0pt]. I will also assume that you've set the option fleqn either at the document class level or while loading the amsmath package.

Equation-specific indentation from the left can be achieved by inserting spacing commands such as \quad and \qquad or, if you need really fine control, either \mkern ("math kern"; this commands takes lengths in multiples of mu, where 18mu=1em) or \hspace, \hskip, and other horizontal-spacing commands.

\documentclass[fleqn]{article}
\usepackage{amsmath}
\setlength\mathindent{0pt}

\begin{document}
\begin{align*}
&1+1=2\\
&\mkern18mu 1+1=2\\