Hi, Hans:

When I used AdobeSongStd-Light as my main font and used
LMRoman10-Regular as the fallback font, I found that the spaces
between Chinese characters and English words are perfect, for example:

\definefontfeature
  [zh][mode=node,script=hang,lang=zhs]
\definefontsynonym
  [zhserif][name:AdobeSongStd-Light][features=zh]
\definefontfallback
  [westernserif]
  [name:LMRoman10-Regular][0x0000-0x0400][force=yes]
\definefontsynonym
  [song][zhserif][fallbacks=westernserif]
\definefont[myfont][song at 12pt]

\starttext
\myfont
中文 English 之间的间隔\par
中文~English~之间的间隔\par
中文\;English\;之间的间隔
\stoptext

But, when I used SimSun instead of AdobeSongStd-Light, the spaces
between Chinese characters and English words became larger and ugly.
In previous example, spaces in the first two lines of text are larger
than those in the third one, even the spaces between English words
also become larger. I remerbered what you said, the fallback fonts can
be independent of the main font. However, I think the spaces between
the words are releated to the main font.

Would you give me a method for solving this problem? The pdf files
used as comparison are attached to in the mail and you can do a
comparative analysis.

Thanks.

by Li Yanrui