于是经过尝试之后，发现关键在于如下代码中获取当前 exe 所在目录并用于得到 config 文件的路径。按这个写法就可以正确输出日志了：

public class Service1 : ServiceBase
{    
    // 进程的主入口点
    private static void Main()
    {
        string assemblyFilePath = Assembly.GetExecutingAssembly().Location;
        string assemblyDirPath = Path.GetDirectoryName(assemblyFilePath);
        string configFilePath = assemblyDirPath + "//log4net.config";
        DOMConfigurator.ConfigureAndWatch(new FileInfo(configFilePath));
        ServiceBase[] ServicesToRun;
        ServicesToRun = new ServiceBase[] {new Service1()};
        ServiceBase.Run(ServicesToRun);
    }
    // 其他略
}