The boy has since recovered, but his mother is charged with attempted murder.
Vanessa Rincon isn't denying the attack, but she has pleaded not guilty by reason of insanity.
In court, Rincon looks like a much different person than she did after police arrested her.
And that's basically her defense.
Her attorney says she suffered a psychotic episode during which she thought the boy was possessed by the devil. Rincon sat calmly next to her attorney Tuesday even as she fought for her future.
The 31-year-old is facing an attempted murder charge for stabbing her 3-year-old son in May 2010.
The first officers on the scene say her demeanor on that day was much different.
"She was saying, 'Kill me. I stabbed my baby. Just give me the lethal injection,'" said Fresno police officer Michael Martin.
Rincon stabbed the little boy more than a dozen times and threw him thrown down a flight of stairs.
Officers say he was in and out of consciousness when they arrived.
They say Rincon was hysterical as emergency medical personnel worked to save her son.
"I told her to calm down, that we were going to help the victim," said Fresno police officer Dean Cardinale. "And she said, 'Don't help him. I want him to die. He's a bad kid.'"
Rincon's attorney admits she attacked the boy.
But he says she was not in her right mind at the time.
For her insanity plea to work, he needs to prove Rincon didn't understand what she was doing or couldn't distinguish between right and wrong.
He focused officers on their own testimony that her behavior was "irrational" and asked them to explain why they felt that way.
"The way she was acting on top of the landing, flailing her arms and the way she acted when I took her into custody," said Cardinale. "She tried to bite my arm."
Prosecutors say Rincon indicated she knew she'd done something wrong when she talked to police at her apartment. They also say she tested positive for meth shortly after her arrest.
The boy is now 4 1/2 and living out of state with an adopted family, including his younger sister who was found unharmed the day Rincon attacked him.
His adopted mother is attending the trial and supporting Rincon.
She showed Action News a picture of the boy taken last week. He looked healthy and happy.