i've located this site on the folder during my root.

/mywebsitedir/

and defined an alias during my apache conf file.

 # Alias for mywebsite

Alias /myalias/mywebsitedir

<Directory /mywebsitedir>
Order allow,deny
Allow from all
</Directory>

now after i make request on my small server.

after i make any request on my small server, as my server is hosting a number of other site.

www.site1.com/myalias

www.site2.com/myalias

www.site3.com/myalias.

i every case it open this site.

i wish to restrict that mywebsite ought to be only obtainable from www.site1.com/myalis .

Thanks

You have to put the Alias right into a VirtualHost for that specific website. Otherwise, it might be a worldwide alias for those websites.