对sum的赋值应该放在循环的外面，且要赋初始值0

<script type="text/javascript">

      var source = "小明:87;小花:81;小红:97;小天:76;小张:74;小小:94;小西:90;小伍:76;小迪:64;小曼:76";

      var source2 = source.split(";");

                var=sum; sum=0;

                      for (var i = 0; i < source2.length; i++) {

                      var score=new Array();

                      score[i]=parseInt(source2[i].substr(source2[i].indexOf(":")+1));

                 //source2[i].indexOf(":")表示从：出现的位置，而+1则是把:下一个开始提取

                      document.write(score[i]+",");

                       sum +=score[i]; }

                       document.write(sum);

   </script>