China's top court overturned Nie Shubin's conviction Friday, in a landmark case that exposed deep flaws in China's criminal justice system.
"I wanted to tell my son: you're a good person, you're innocent," Zhang told CNN.
Amid emotional scenes in the courtroom, judges ruled that Nie's original trial didn't "obtain enough objective evidence," saying there were serious doubts about the time of death, murder weapon and cause of death.
"The moment the justice announced the verdict, Zhang burst into tears," lawyer Li Shuting said. "Not only her, other people in the courtroom felt emotional too. I almost stood up and applauded but I didn't, I had to comfort Zhang as she was crying so hard."
Another man, Wang Shujin, confessed to the crime that Nie was executed for in 2005 -- 10 years after Nie was executed.
"In this case, the Supreme People's Court changed the original sentence and ruled Nie Shubin is innocent," according to state-run news agency Xinhua.
"As the evidence is not accurate or adequate and nor can it exclude the possibilities of other suspects, it is recommended that the Supreme People's Court retry the case."
"After his death, his mother repeatedly dreamed of him knocking at the window and muttering 'Mom, I'm back'," Li said.
Nie would have been 42 this year. Following his execution his father Nie Xuesheng tried to commit suicide, but survived.
"My son can finally rest in peace," the older Nie told the Beijing Times Friday.
Mother fights for her son
In July, Zhang told CNN she was very excited about the review.
"I'll visit his grave soon to tell him that mom's efforts all these years weren't in vain -- and justice will prevail in your case," she said.
The court said that Nie's family could ask for compensation from the government.
CNN first met Zhang in her small village in 2011, when the farmer from Hebei Province was still fighting in earnest to clear her son's name, making countless journeys to the provincial capital of Shijiazhuang -- some 320 kilometers (200 miles) southwest of Beijing.
Her refusal to stay quiet appears to have made a difference in this case.
Authorities tried Nie behind closed doors and barred the parents from the courtroom, but Nie told a lawyer hired by his family that he was beaten into a confession on his sixth day in jail.
Seven months after he was first detained, the government executed Nie -- without notifying his parents.
After the initial shock, Zhang had to endure more agony to locate her son's remains and deal with a failed suicide attempt and subsequent half-paralysis of her husband, who was devastated by Nie's execution.
For years, it seemed no one would listen, but Zhang later found an unlikely ally in the People's Daily -- the official newspaper of the ruling Communist Party. It ran a scathing commentary in September 2011 that asked: "In a case where someone was clearly wronged, why has it been so difficult to make it right?"
"Rehabilitation means little to the dead, but it means a lot to his surviving family and all other citizens," the paper said. "We can no longer afford to let Nie's case drag on."
Many have viewed Zhang's plight -- and the case involving her only son -- as an egregious example of widespread police torture, deficient due process and lax review of death sentences.
China is the world's most prolific executioner,
killing thousands of people last year, according to rights group Amnesty International.
In 2013, China's top court banned the use of torture to extract confession
amid widespread concern that it was being used to wrap up cases quickly.