A short history of the Thai flag was given under the paper History of the Thai Flag. Prachumphol also showed us that Thailand recently obtained the title for the world's biggest flag in 2016. Since the Thai flag had reached its 100th year of use in its modern form, his group was responsible with creating worlds largest flag to celebrate its centennial. Thus on November 30th 2016, Thailand became the nation with the largest flag in the world. In the image below you can see the world's largest flag being hoisted on three flag poles. His speaking partner is holding the Guinness World Record's certificate.
The Thai flag has undergone many changes. But the current flag was adopted with the middle blue stripe when the Thais entered on the side of the allies in 1917 during WWI, thus the flag you seen now is the flag modern flag of Thailand.