Kovrig was detained by China's spy agency on suspicion of "activities endangering national security" on December 10, days after Canada arrested a Huawei Technologies Co. executive as part of a US -led extradition effort.
The court had originally found Schellenberg guilty of drug smuggling back in November, sentencing him to 15 years in prison and fining him 150,000 yuan ($22,000).
At Schellenberg's appeal hearing last month, prosecutors said that emerging evidence indicated he had played a bigger role in a drug trafficking network and so his initial sentence was too light.
The ruling has deepened a rift between China and Canada, with the sentence coming on the back of China's discontent with Canada's arrest of a top executive from Chinese telecom giant Huawei in December on a United States extradition request related to alleged violations of United States sanctions against Iran.
Washington wants Huawei executive Meng Wanzhou - the daughter of the company's founder - extradited to face charges that she misled banks about the company's business dealings in Iran.
"This is a case about Xu Qing, he is an worldwide drug smuggler and a liar", Schellenberg told the court.
"This is a very unique case", Zhang told The Associated Press.
In an unusual move, China's propaganda office invited foreign reporters to attend both Schellenberg's hearing and retrial.
Huawei chief financial officer Meng Wanzhou was arrested in Canada, sparking a backlash.
A Canadian man who was convicted of smuggling drugs in China has been sentenced to death.
The case has been condemned by Western legal experts and Schellenberg's relatives, who say that China is using Schellenberg as a bargaining chip in its efforts to free a top technology executive detained in Canada.
Before his arrest, on December 1, 2014, Schellenberg had prepared to flee to Thailand from Dalian but was arrested when in transit in Guangzhou, the court said.
Two other Chinese men were involved in the case - one was sentenced to life in jail, while the other was given a suspended death sentence. "It's clear that Chinese courts are not independent, and by systematic design, courts can be influenced by Communist Party officials", he said.
In 2009, China executed a Briton, Akmal Shaikh, on charges of smuggling heroin despite his supporters' protest that he was mentally ill.
Hua, the foreign ministry spokeswoman, called on Poland to allow Chinese diplomats to visit the suspect, Wang Weijing, and handle the case in accordance with the law.
Kovrig is not now serving as a diplomat and had entered China on his most recent trip on a regular passport and business visa, she said.
The Chinese media began publicizing Schellenberg's case in December after Canada detained Meng, who faces extradition to the US on fraud charges.
"The relevant people on the Canadian side should seriously learn and study the Vienna Convention on Diplomatic Relations". At least 13 Canadian citizens were detained in China since the CFO's arrest, the Globe and Mail reported earlier this month.
China's ambassador to Canada accused the country last week of "white supremacy" in calling for the release of the two Canadians while describing the detentions as an "act of self-defence".
At a news briefing Friday, a spokesman for the ministry, Lu Kang, said critics should not undermine Chinese law for political purposes.