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.
Ma's home village has also been isolated and thoroughly disinfected.
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.