Thanks @Heliosurge @Enopho
It may be something wrong with Firmware update. Please operate again as below:
Firstly press all 3 buttons in 8 second, and then loose power button and keep other 2 buttons in 8s, and then loose middle button and keep left one in 8s, you could loose the final one after 8s more.
PS: I also made a small video about operation above for your reference, link at: https://mega.nz/#!pdxWhLyK!tsQN2XYRcuHjc7v4beQ9_d5s10K0E3Rn84vYf9ijVpQ
Edit 30/08/17: Sorry for confused expression above.
Please firstly press all physical buttons at the same time (do not connect the headset), and then just plug the USB cable of the headset to a port 2.0 / 3.0, and then loose power button after 8 seconds, meanwhile keep other 2 buttons in 8s, then loose middle button and keep pressing the last button in 8s,
then you should see the light is flickering, it means the headset enters to DFU (Firmware update) mode. If not, please operate above again.
When the light on the helmet is flickering, please download firmware update tool to flash it, link at: http://forum.pimaxvr.com/t/attention-pimax-official-firmware-manual-update-tool/2160