It is advisable for you to get a L visa to China in Belgium, a single entry L visa is valid for 3 months, it won't expire when you go to China after visiting Thailand. It is hard to say that you surely obtain a L visa in Thailand, the visa policy changes every time, maybe you can get it in Bangkok as a tourist, but maybe your application will be denied by China embassy in Bangkok because you don't work/reside in Thailand.

Thank you for your answer,
Actually, I'm not in Belgium, I'm travelling in Australia and Asia, that's why I was wondering if I can get the L visa in Thailand.
I understand if I can't get it from Thailand...
What about Hong Kong? Do you think I can get the L visa there as a tourist?

You can apply for China visa in Hong Kong and it is easier than to apply in Bangkok.