Hello MURUGAN ,

You can follow below steps for show watermark text on all page in qweb report:

Prepare custom report_external_layout_header template ,add  <div class=”header” > tag  in your qweb report template
or
you can add <div class=”header”>tag  before <t t-set="doc" t-value="doc.with_context({'lang':doc.partner_id.lang})" /> tag in your custom qweb report :
xml code :

      <div class="header" >
            <!-- Company Header Details-->
            <div 
style="position:absolute;opacity:0.25;z-index:1000;transform:rotate(300deg);-webkit-transform:rotate(300deg);width:150%;">
            <p style="font-size:50px;">WATERMARK TEXT</p>
            </div>
        </div>