are you speaking about the "subjects" for the articles, as imported in the native article-XML element <subject>? Or are you referring to the indexing of the abstracts and full-text search keywords?
If you have problems with the full-text and abstract keyword indexing, you might find some information here:http://pkp.sfu.ca/support/forum/viewtopic.php?f=8&t=4283
The keyword indexing function of OJS is not very suitable for indexing content in Chinese, because it relies on spacing between words, which is not common in Chinese. Therefore it would usually index long phrases or even whole sentences, if you're using it for Chinese.
Maybe you would also like to have a look at the new Lucene plugin to expand indexing and full-text search capabilities:https://github.com/jerico-dev/ojs/tree/dev-stable/plugins/generic/lucene
This Lucene plugin comes with OJS 2.4.x. I believe this will handle Chinese language content better.
Kai Weber (韦博开)