I create a new NPAPI plugin and copy the dll file to mozilla plugins directory, but at this time, Chrome can't find the plugin when load a page with embed associate with this plugin mimetype. Then after I open a page of 'about: plugins', that works, so Chrome will refresh plugins list when requested 'about: plugins' and found the new plugin.

But how can a plugin installer call the plugins-refresh method so that let Chrome found the new plugin?

I don't know why it wouldn't find it on browser loading, but find it on going to about:plugins. O.o
This describes how to use NPAPI from a regular extension, which I believe is the recommended method for applying them, as opposed to the registered type thingy flash and stuff do.
Sorry it's not exactly what you're looking for but it's all I've got. >_>