To get a good IELTS score, you need to use the kind of language that native speakers might use. This doesn't mean 'difficult words', it means good groups of words (phrases and collocations).
When reading, try to look for phrases that English speakers use. Here's an example paragraph from the Economist Magazine with a few useful words and phrases underlined.
It is always a little disconcerting to realise a generation has grown up never knowing what it was like to manage without something that is taken for granted today. A case in point: the World Wide Web (WWW), which celebrated the 20th anniversary of its introduction last Saturday. It is no exaggeration to say that not since the invention of the printing press has a new media technology altered the way people think, work and play quite so extensively. With the web having been so thoroughly embraced socially, politically and economically, the world has become an entirely different place from what it was just two decades ago.