From Siam Legal.
Qualifications of Thai Marriage Visa
Must be married to a Thai national
This is substantiated through a Marriage Certificate (Kor Ror 3) and Marriage Registration (Kor Ror 2). If the applicant got married abroad, the applicant or the Thai spouse must report the marriage to the amphur and obtain Kor Ror 22.
Must meet the financial requirement
Security deposit of THB 400,000 in a Thai Bank Account for at least 2 months prior to the visa application; or
Monthly income of at least THB 40,000. A letter from the foreigner embassy has to be shown to verify this income
Supporting documents as proof of the security deposit in a Thai bank are as follows:
Updated bank book or passbook
Bank letter stating that the money had been deposited to the account from an overseas source for not less than 2 months.
Other documents that may be required to be presented:*
There are instances depending on the immigration office when a copy of an Affirmation of Freedom to Marry is also required. This affirmation is obtained by the foreigner from his foreign embassy in Thailand.
For the foreign nationals who are unable to obtain income affidavit from their respective embassies, they can show monthly income in a form of 12-month bank statement of their Thai bank account showing monthly deposit of THB 40,000 per month.