body,html {height       : 100%;margin       : 0;padding      : 0;
}
#view {width        : 100%;height       : 100%;position     : relative;text-align   : center;
}
.watch {width        : 98% ;height       : 30px;border       : 1px solid #00a5ba;outline      : none;border-radius: 4px ;
}
.fixed-btn {width        : 100%;height       : 40px;background   : #00a5ba;border-radius: 4px ;position     : absolute;bottom       : 0   ;left         : 0   ;
}

<div id="view"><input type="text" class="watch"><div class="fixed-btn"></div>
</div>

window.onload = function () {

　　var load = document.body.clientHeightvar view = document.getElementById('view')
　　view.style.height = load   'px' 

}