Tsinghua University did not issue a diploma to Sun Wei because she was a suspect in the thallium salt poisoning case. Sun Wei was suspected of being the only person who could obtain thallium salts and get close to the victim Zhu Ling at the same time. The police investigated her but was later released. Sun Wei has a prominent family background, which is easy to check on the Internet. Certain actions of the police in handling the case caused the public's perception of the incident to change into the "Zhu Ling-Sun Wei case."

Although in 2005, Sun Wei issued a clarification statement in the Tianya community through a channel called "Sun Wei Statement", denying that it was related to Zhu Ling's poisoning. She claimed that she was not the only student exposed to thallium, and Tsinghua University's reagent management was not strict. She said she had no conflict with Zhu Ling and had no motive for poisoning. According to "Xinmin Weekly", Zhu Ling's roommate Sun Wei may have been exposed to thallium salts due to her project, and the number of people in Beijing exposed to thallium sources was limited at the time, which made her considered to be eligible for poisoning.

In 2013, Sun Wei once again issued a statement in the Tianya community, expressing her regret that the truth had been buried and her strong desire to bring the real culprit to justice. Her attitude showed the impact of the case on her life. . However, these clarifications did not change her close connection with the case, so Tsinghua University did not issue her a diploma.