Speaking on LBC 97.3, the mentor and judge denied that a sing-off was added to Sunday's results show to ensure that Cher Lloyd stayed in the competition instead of his own act Mary Byrne.
Walsh said: "I'm furious about the rumours because it's all untrue, it is all untrue. I can understand why people are angry but I'm Mary's mentor. I wanted her to make the final but I knew I was in trouble, I so knew I was in trouble on Saturday night."
Asked if he thought Byrne would have stayed in the competition based on the viewers' votes alone, he replied: "I don't know. We never get the vote until after the series. We get them after the final. I don't know. I honestly don't know.
"I was hoping that Dannii [Minogue] might have given her the chance to do that, but Dannii just thought she was doing the right thing by picking Cher. I wanted Mary in the final, but I knew I was in trouble... I was canvassing for votes all week and I was doing lots of press stuff."
Of the change from last year's semi-final, which went to the public vote, he continued: "Simon Cowell knows how to make a TV show and every week he wants to change something. It's never predictable, it's never like it was the year before.
"Some weeks there might be two eliminations. Even with putting in the wildcards. That was the problem because we had 16 people this year instead of 12. We have four people in the final this week. It was never going to be like any other year."
Walsh added: "There's nothing fixed. Simon might be a lot of things but he doesn't fix the show. He might be arrogant and pompous and vain and all that, but he doesn't fix the show."