I am Bangladeshi and living in Thailand with dependent visa. I want to attend the conference in China. They will provide Invitation card for this. What should I do?

You can submit all the documents to the embassy in Thailand and wait for approval. Or you may assign a local agency to handle everything for you. Except the invitation letter, you should provide your passport, a filled application form affixed with a passport size photo and a copy of your passport info page and Thailand visa pages. Sometimes, the applicants are required to provide return air ticket and hotel reservation orders and financial proof.