参考了一下楼上大神的，再对比一下您的，我发现你没有声明result

两种解决方案：

一、

<script type="text/javascript">
    function count(){   
        var a=document.getElementById("txt1").value;
        var b=document.getElementById("txt2").value;
        var c=document.getElementById("select").value;
        var result=document.getElementById("fruit");
        switch(c){
            case "+":
                result.value=parseInt(a)+parseInt(b);
                break;
            case "-":
                result.value=parseInt(a)-parseInt(b);
                break;
            case "*":
                result.value=parseInt(a)*parseInt(b);
                break;
            default:
                result.value=parseInt(a)/parseInt(b);
                break;
        }
    }
</script>

二、

<script type="text/javascript">
    function count(){   
        var a=document.getElementById("txt1").value;
        var b=document.getElementById("txt2").value;
        var c=document.getElementById("select").value;
        var result=0;
        switch(c){
            case "+":
                result=parseInt(a)+parseInt(b);
                break;
            case "-":
                result=parseInt(a)-parseInt(b);
                break;
            case "*":
                result=parseInt(a)*parseInt(b);
                break;
            default:
                result=parseInt(a)/parseInt(b);
                break;
        }
        document.getElementById("fruit").value = result;
    }
</script>

你应该看完代码后就能明白自己错在哪里了