The former cricketer explained that he began putting weight on during the beginning stages of his sporting career and soon found himself purposely throwing up his meals.
"I used to be a really skinny kid, I used to be tall and skinny and I didn't drink, I didn't do much," he said.
"Then I started putting weight on. My lifestyle was great, granted, when I started living on my own. I got to about 19 and a half to 20 stone.
"It was all a bit deeper than that because to lose the weight, I wasn't really sure how to go about it. I started being sick, I was sick a lot. I would eat and I would be sick.
"I would be doing it myself. I would put my fingers down my throat and everyone within the team hierarchy thought it was great because I was losing weight."
Flintoff continued: "I was in India, I was in New Zealand, my weight was coming down. I was doing the training - but I wasn't eating.
"I've thrown up in cricket grounds around the world in restaurants, all sorts and I lost about 15kg by doing that.
"It's not the right way to go, which I am very aware of. You start off doing it if you have a bad meal, you eat something you don't think you should have eaten or you have drunk too much.
"Then before you know it, even good meals you are getting rid of. It becomes a real habit. The only way really to lose weight is to train more and eat less, eat the right things and that is what I am going to have to do on this."
Flintoff made the revelations in his upcoming Sky1 documentary Flintoff: From Lords to the Ring, which will show his training to become a heavyweight boxer.
He added that he hopes to spread awareness about his bulimia struggles, saying: "I don't have qualms about speaking about it, it is something that happened a while ago now.
"And I suppose if anyone was feeling like that, it is all right to talk about. I learnt that speaking out helps.
"Maybe if someone is watching and feeling the same way, it's actually people have been through it and you can talk about it and that's the way in which people can help."
Mike Tyson will also make an appearance in the programme, which will air on Sky later this month.