It may sound naive.. Is there any good open source FPGA design? for education purpose

The short answer is that there is not any equivalent to the Linux kernel, and there is not anything publicly available that people point to as a gold standard for HDL techniques.

You probably won't learn much without picking a specific objective or application domain (networking, video, storage, ... ) and trying to write something yourself. And then for guidance, fall back to traditional methods such as books and papers.