Huaneng Power International (SHA:600011, HKG:0902) said it experienced an unusual stock movement over three consecutive trading days, but its operations are still normal, according to a Hong Kong bourse filing on Monday.
The Chinese power company's Shanghai shares slumped 6% during Tuesday's midday trade.
Huaneng's closing price between May 28 and June 1 was over 20%, exceeding the growth of the bourse's threshold.
The company said it has no significant announcement or undisclosed information that could have affected its stock price.