html: 
<div class="canvas" id="cas1">
                                <canvas class="c2"  id="canvas1" style="position:absolute;" width="47"> </canvas>
                            </div>

js:

$(function () {
    var winH = $(window).height();
  
    $(".musicbg .colnumbg .item1 .canvas").height(winH * 187 / 644);
   
  
    //讓canvas的高度等於父親的高度
    var height = $(".musicbg .colnumbg .item1 .canvas").height(winH * 187 / 644);
    var height = $('.canvas').height();
    $('#canvas1').css("height", height);
    var width = $('.canvas').width();
    $('#canvas1').css("width", width);
});