Thanks Sergey Kandaurov.
The second problem is that I set grpc_read_timeout and grpc_send_timeout but it seems does not take effect. I tested many times and found that if the read_timeout less than default 60s, then it works. But it has no effect when read_timeout more than 60s, nginx will automatically close the connection between nginx and grpc server by every 60s. How to configure parameters to fix this issue?