The case was reported in Subei County under the jurisdiction of Jiuquan City.
The man named Ma, a herder, died on Wednesday in hospital.
The provincial health and family planning commission confirmed the case after lab reports, state-run Xinhua news agency reported.
Ma grazed his sheep in a pasture where widespread plague has been reported among marmots in recent years.
A total of 41 people who had close contact with Ma have been put in quarantine under medical observation. None have shown any symptoms so far.
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It is the second plague death in Jiuquan this year. The other fatality was also marmot-related.
The National Health and Family Planning Commission has sent specialists to the county to prevent the plague from spreading.
Plague is categorised as a Class A infectious disease, the most serious under China's Law on the Prevention and Treatment of Infectious Diseases.