Is cheap labour cost the only reason for Chinese product to be competitive in the world market or is there any other reasons?

Not only labor costs which affects the final price, but also other factors like:

- Export regulations

- Innovation and variety of products

- Flexibility in production policies of most manufacturers

- Mass production

 

Major factor is China's national government policies provides subsidies and special status for export. Like dumping , lower currency value and MFN status from almost every country. 

Cheap labour, subsidies, easy availability of resources for mass production( traditionally mass production was considered a result of high . mechanisation, not in the Chinese model, its a good fusion). Then lower currency, access to high volume dumping markets like India, Africa, and they have the scope to just produce and sift through for high quality rather than manage efficiencies.

CHEAP LABOR IS NOT THE ONLY REASON BUT IT IS ONE OF THE REASONS.