Beijing is concerned a war between North Korea and America will put towns and villages near the northern border at risk.
Chinese officials also fear an influx of millions of North Korean refugees crossing into China if all-out war breaks out.
Beijing has increased military patrols along the border with North Korea, locals in Chinese villages have reported.
Radiation monitors have also been introduced to vulnerable towns to enable an early response to contamination in the event of a US nuclear strike on the hermit state.
And citizens, already discouraged by the China state from helping defectors, have been reminded of their duty to hand in escapees, who are then returned to North Korea to face punishment.
Any Chinese national who helps a North Korean defector risks arrest and punishment, despite the horrendous conditions suffered on the other side of the border.
A banner posted to a border fence at the Chinese town of Dandong said: “Citizens or organisations who see spying activities must immediately report them to national security organs.”
Citizens have noticed a growing military presence along border regions as the war of words between Donald Trump and Kim Jong-un escalated last month.
And as Kim and Mr Trump take turns to boast about their “nuclear buttons”, Chinese authorities are taking no chances – with the effects already noticed by those living near the border.
One Chinese fisherman said: “Before, the North Koreans came to our side to fish. Now they don’t dare. The army patrols and watches.”
READ MORE: Will the USA attack North Korea?
And a retired Chinese worker at the Sup’ung hydroelectric dam said border security was increasing and defectors were not being tolerated.
He said: “The border is tightly controlled now. If they do come over here, we’ll catch them and send them back.”
A professor at Peking University also revealed “radiation environment automatic monitoring stations” were now being installed along the border.
He said: “If the monitoring stations show any abnormalities, we will immediately alert citizens.”
And last month a state-run newspaper near the border published a illustrated guide on how to protect yourself in a nuclear attack.
It said: “If there is a river, lake or pond near you, jump in to protect yourself. Then flush out your nostrils, rinse your mouth, and clean out you ear canals.”