用margin吧,layout_width用fill_parent
<EditText
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:layout_marginLeft="20px"
    android:layout_marginRight="40px"          
 /> 
                       |---------------------------|
             |               View        |
      margin |   padding                 |
      <------|--------->                 |
             |---------------------------|