Muang Thong came from 0-2 to draw level at 2-2 to force an extra-time but neither side could score a goal.
After three all in the shoot-out, Chiangrai goalie Chatchai Prombutr denied a kick from Siroj Chatthong. Chaiwat Buran nailed his shot to propel Chiangrai to the final with Bangkok who blanked Chiang Mai United 3-0.
The final is due on November 25 in Bangkok.