start:
    do
      {
          j=0;
          if(strlen(ch1)&&b)
          printf("\n  密码长度错误，请重新输入密码:");
          b=1;
          while((ch1[j]=getch())!=13)//检测输入字符的最后一位是否为回车键
          {
            if(j==4)
            {
                printf("*");
            }
            else
            {
                j++;
                printf("*");
            }
          }
          ch1[j++]='\0';
      }while(strlen(ch1)!=strlen(ch2));    //循环检测输入密码的长度   
      for(;i>0;)
      {
         if(strcmp(ch1,ch2)==0) //判断密码是否正确
         {
             printf("\n\n  登陆成功,欢迎使用泊车自助缴费系统!\n\n");
             return 1;
         }
         else
         {
            i--;
            if(i==0)
            {
            printf("\n\n  密码错误,登陆失败!\n");
            exit(1);
            }
            else
            {
             printf("\n\n  密码错误!");
             printf("请重新输入管理员密码(您还有%d次机会):",i);
             b=0;
             j=0;
             goto start;
            }   
      }
    }
}

你把上面do while这一段复制到goto start这里试试