A man who sexually abused his young stepdaughter has been jailed for at least seven years.

The man, 36, pleaded guilty to persistent sexual abuse of a child and possession of child pornography.

The Adelaide District Court heard the abuse occurred from when the girl was 10 until she was 13.

Judge Michael Boylan said the man had long been socially isolated and struggled to make friends or have relationships with adults.

The hearing was told the man felt more confident with his stepdaughter than with his wife.

"Your difficulty forming relationships with adults explains in part why you turned to your stepdaughter for physical comfort. She made you feel happy and worthwhile," the judge said.

"You told the psychiatrist you felt more confident with her than you did with her mother, your wife.

"You fell in love with your stepdaughter and enjoyed your relationship with her."

Suicide risk

Judge Boylan said the crimes were a gross breach of trust.

"Although you do not have good insight into the effects of your behaviour on [the victim], you have taken full responsibility for what you have done," he said.

"As a result you are now more isolated than before and have lost all self-esteem and will be a suicide risk in prison.

"The sexual offending against [the victim] is at the high end of the category of this sort. It was frequent and there was sexual activity almost daily towards the end. I am not going to go into the precise details of the offending, they are too appalling."

The judge said he doubted the girl could yet comprehend the likely ongoing effects.

"It was a gross breach of trust. You were her stepfather and, in her eyes from a very young age, you had effectively been her father. I doubt that she yet knows or can know the damage that you have done to her. Sadly I suspect that she will never get over it," he said.

"General deterrence is extremely important in matters such as this. The law, in imposing high penalties, must make it plain that behaviour such as yours in our community and in our families cannot be tolerated."

Judge Boylan said had it not been for the man's guilty plea a head sentence of 14 years would have been imposed.

He reduced that to 10 years, with a non-parole period of seven years.