I like to display only 20 lines of post with the read more link using
the_content( __( 'Continue reading <span class="meta-nav">→</span>' ) );
But it shows full content . How can i strip it .please help me !
You need to add a more quicktag to your content
<!--more-->link to each of your posts, at the appropriate location (as suggested by @spartacus)
the_contenthook, to return the appropriate amount of content (I'm not sure how you would filter to return 20 lines of content), plus a read more link
the_content(), and filter
excerpt_lengthhook, to return the appropriate number of words