I found the problem.
I used the wireshark to analyse the packets between nginx server and grpc server and I found that in the last packet from grpc server to nginx client, there are five frames,SETTINGS, HEADERS, DATA, HEADERS, WINDOW_UPDATE. The second HEADERS set the 'end_stream' and ngx_http_grpc_filter checked it
(about the 2100th line in the ngx_http_grpc_module.c)
if (ctx->end_stream) {
u->length = 0;

if (ctx->in == NULL
&& ctx->out == NULL
&& ctx->output_closed
&& b->last == b->pos)
{
u->keepalive = 1;
}

return NGX_OK;
}
everthing is true but b->last == b->pos
because there is still a WINDOW_UPDATE frame in buffer and the difference between b->last and b->pos just is the length of WINDOW_UPDATE frame.
That's why keepalive in the upstream not work, it is not set by the grpc module.
Please check the bug. The keepalive problem also happens between grpc client and nginx but it is not so serious like this one but it needs your attention, too.