Hi all,
I am the OP of this post. Thanks for those who replied last time.
As I described last time, I'm the new admin of the lab website service.
The former admin has installed the ESXi 5.5 host on a ASUS PC(not under VM).
We run our web services in the VMs managed by vcenter.
However, after we shutdown the server for electricity device check, when we reboot the server, at first everything is fine as usual.
But after 2 days, our server's power is off unexpectedly. The ESXi host will finish setup(It will show ip and DN url and show I should download tools to manage). After about 5 to 10 minutes, the power of the server shutdown unexpectedly.
In the beginning, I thought the problem is from hardware. But it seems power supply, motherboard and hard disk are all fine.
When I finally know how to use terminal on ESXi host, I found that before the server shutdown. The command line will show message:
"vmx| W110: Caught signal 15 -- tid 36627 (eip 0x133597b8)"
As the following photo shows:

I've read that signal 15 is a SIGTERM to shutdown process. But I have no clue how to troubleshoot this. Really need some veteran advice to guide me how to find the root cause and solve this.
Thanks a lot!
Also, I am wondering is it possible to use a external usb-connected drive with other version of OS to toggle the original local disk?
If it's possible, does that mean that I can use a later version of ESXi host and get the old VMs in the local disk and run them on the new OS properly?