Liu Yanfei, from Harbin, capital of Heilongjiang province, was repairing a dysfunctional pump yesterday when he inhaled toxic fumes while working, causing him to faint and fall into water.
Several of his colleagues attempted to save him, but they also fell in water.
Two of the workers were later rescued, while five others, including Liu, died.
Rescuers suspect that the fumes were methane. A thorough investigation has been launched into the case, state-run Xinhua news agency reported.
Many northern cities in China experienced heavy rain resulting in waterlogging.