I'm holding Thai passport. And have planed to joint company for meeting in Beijing for < 2weeks.

Yes, you need to apply for a visa to Beijing for the meeting. The regular processing at the emabssy takes 4 working days. For this purpose, you need to show the invitation letter from China. Other basic requirements are your valid passport as well as a copy of its information page, one neatly completed Visa Application Form, one recently-taken color photo, invitation letter from the meeting organizer in China, or an introduction letter from your Brunei companies, etc.
Besides, you need provide the original and a photocopy of certificates for permit for residence, work, study or valid Brunei visa.