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When I run free -h, I get:

 total used free shared buffers cached
Mem: 251G 208G 43G 179G 144M 190G
-/+ buffers/cache: 17G 233G
Swap: 5.6G 5.6G 1.1M

How can shared memory(179G) + cached memory(190G) exceed total memory (251G) ? Can memory be count as both of cached memory and shared memory?

All shared memory is also counted as cached.

shared memory is implemented using tmpfs internally. tmpfs is implemented as a thin wrapper for the page cache, just without having any backing store (except that tmpfs is swappable).

man free does not explain this. At least on my system (provided by procps-ng, last updated 2016-06-03). Sorry. All man free tells you is that cache is taken from Cached in /proc/meminfo. If you read man proc (from man-pages, updated 2017-09-15), it has documentation for the fields in the meminfo file, but it still fails to inform you that Cached includes Shmem.

You can see this with an experiment. While you have free memory (not available), you can create a file in a tmpfs, for example:

dd bs=1M count=100 < /dev/zero > /dev/shm/test.tmp

The result of this is that both the shared and cached figures in free -m increase by 100.

If you only have available memory, and do not have enough free memory to test this properly, you can create free memory by dropping as much page cache as possible, by running echo 1 | sudo tee /proc/sys/vm/drop_caches. Of course dropping page cache can be very bad for performance. Do not do this on a real server :-).