The 36-year-old reeled off half-centuries in each of the first five frames as he looked set to whitewash his Thai opponent.
A missed red to the middle on 46 delayed the inevitable, and after Saengkham had cleared to get a frame on the board, Allen won the next to wrap up victory in under two hours.
"I'm delighted with the way I played, I was pretty much flawless for the first four frames," said Allen afterwards.
"After that I kept making it tough for him and didn't give him easy chances. I think I only missed one ball.
"I remember playing John Higgins in the (2013) World Open semi-finals, it was 2-2 at the interval and he didn't pot a ball in the next four frames. That's pretty much how it was today, it was nearly the perfect performance."
Allen will face Ryan Day in the final after the veteran Welshman held his nerve in a final frame decider to edge out world number 61 Robbie Williams 6-5.