html

<div class=mod clearfix>               
       <div class=ml >         
           <p>左侧定宽</p>        
       </div>              
       <div class=mr>
           <div class=modcon>
                <p>右侧自适应</p>
           </div>
       </div>              
   </div>

css

.ml{float:left; width:190px;margin-right:-190px;}
.mr{}
.mr .modcon{margin-left:210px;}