According to the witness, the second-year student, identified only by his surname Lin, was talking on the phone for more than 10 minutes before his death. "I heard him say 'do not force me' and 'give me two more days and I'll return you the money'," the witness said.
"I saw him hang up and stand up and then all in a sudden he just disappeared," the witness added.
Though Lin was immediately rushed to hospital in Panyu in the southern province of Guangdong, doctors were not able to revive him.
Another student described as a classmate of Lin's said the gambler had lost nearly 20,000 yuan ($3,200 or N522,000) in bets on several World Cup games.
"We heard that he borrowed quite a lot of money and the interest rate was rather high," said the student.
It did not specify which teams Lin had backed.
China has millions of football fans, some of who are both devoted viewers and passionate gamblers.
Sales of a government-run World Cup-related lottery reached four billion yuan (104 billion naira) by Saturday.