Can I submit the paper to arxiv and the conference at the same time? I think the conference where the paper is submitted will have 10 days to start submitting the abstract. There are 23 days before the papers start to be handed in. But my thesis will be completed within a week. The review of the paper takes nearly two months. May I be worried that other people will publish papers with the same idea during the review period.
Can I upload the paper to arxiv now? Then submit it to the conference normally after 20 days?
There is a sentence in the conference request:

The only exceptions to the above rules are for the authors’ own papers
in (1) workshops without archived proceedings such as in the ACM/IEEE
digital library (or where the authors chose not to have their paper
appear in the archived proceed- ings), or (2) venues such as IEEE CAL
or arXiv where there is an explicit policy that such publication does
not preclude longer conference submissions. In all such cases, the
submit- ted manuscript may ignore the above work to preserve author
anonymity. This information must, however, be provided on the
submission form – the program chairs will make this information
available to reviewers if it becomes necessary to ensure a fair
review. As always, if you are in doubt, it is best to contact the
program chairs.
Worrying about other people publishing your paper isn't something you should do. One, it's not in your control, that'll happen if it has to happen. Two, even if it does, there are always other parameters which can differentiate the paper.
This conference says you can post it to Arxiv, so go ahead! But I don't really think it'll stop someone else from publishing their material else where. Especially when the pre-print upload date is so close to the conference many might not even realize that their is a paper with the same domain!