If the bottom of a character close to the Win Descent (such as ‘g’), the bottom of the character will be cut in some scales.
e.g.: 1. create a font contains a character ‘E’
2. move the Win Descent close to the bottom of ‘E’ and save
3. apply this font in Microsoft Word.
4. type ‘E’ in Microsoft Word using a large font size (such as 36, 48)
5. try to zoom out with different small scales (below 50%, such as 35%,30%,25%… although the character is very small)
You may see that the bottom of ‘E’ was cut, it seems ‘F’. 
This problem also happens if the top of character close to the Win Ascent.
I know this problem will be solve if we leave space between the Win Ascent/Win Descent and the character, but we want to make the chacracters close to the Win Ascent and Win Descent in our application. 
What is the minimum space is needed to display normally in different scales 
As far as I know this only occurs when your font has set it’s Contents and Layout version to Version 0. Change it to Version 1 or higher, and make sure you’ set the appropriate Unicode and Code Page ranges (the Calculate buttons will do the work for you). Do let us know your results.
Thank you for your suggestions. I have tried your suggested settings, but the problem still happens.
Please find the detail settings of the font and the pictures of showing the display problems in the attached files.
I don’t think this is related to your initial problem where you talked about parts of glyphs cut near Win Ascent/Win Descent.I think this is more likely due to the lack of hinting.
I tried many different setting and hinting of the font, only keep the WinAscent closing to the top of the character.
This problem still happen. So I tried to see whether this problem happens in other fonts.
This problem also happens in Arial. I only move the WinAscent to the capheigh, kept other settings and hinting of Arial. This problem still happen.
Is there a minimum required distance between the top of character and WinAscent for displaying normally?
I don’t know the internals of Microsoft Word, so I can’t help you with that. I do know more sophisticated DTP software like Adobe InDesign do things a bit different, so you might not see this unwanted behavior there.