In order to apply to become a Thai Permanent Resident, you must meet the following criteria:
You must have had a Thai non-immigrant visa for at least three years prior to the submission of your application. Holders of multiple NON-Immigrant visas can not apply. You must have 3 consecutive yearly extensions in order to qualify.
You must be a holder of a non-immigrant visa at the time of submitting your application.
You must be able to meet one of these categories to apply for PR status in Thailand:
Investment category (minimum 3 – 10 Mil. Baht investment in Thailand)
Working/ Business category
Support a family or Humanity Reasons category: In this category, you must have a relationship with a Thai citizen or an alien who already posses a residence permit as a husband or wife; father or mother; or a guardian of a Thai child under 20 years of age.
Expert / academic category
Other categories as determined by Thai Immigration
PLUS:
You have to be at least 18 years old
You must have a clean criminal record
You must be employed in the country
It is compulsory that you know how to speak and write in Thai
You must know how to recite the Thai National Anthem
It is required that you get 50 points in the Points Based System