你的最后一个else应该写在

if(checkname.charCodeAt(i)<=225)

这个判断的后面吧：

var checkname = document.getElementById("name").value; //读取用户输入信息
        if((checkname == "") || (checkname == null)) { //判断用户名是否为空
            var name = "请输入用户名！";
            yonghuming.innerHTML = name;
            form1.name.focus();
            return;
        } else if(checkname.length < 2) {
            name = "请输入正确姓名！"
            yonghuming.innerHTML = name;
            form1.name.focus();
            return;
        } else if(checkname.length >= 2) {
            for(var i = 0; i < checkname.length; i++) {
                if(checkname.charCodeAt(i) <= 225) {
                    name = "请输入姓名的正确格式！";
                    yonghuming.innerHTML = name;
                    form1.name.focus();
                    return;
                } else {
                    name = "✔";
                    yonghuming.innerHTML = name;
                    return;
                }
            }
        }

像你写在外面的很明显的是逻辑错误嘛