An inspection of companies based around Beijing found more than 70% were violating air pollution regulations, Chinese state media says.
Firms pumped out more emissions than allowed, operated without licences or had insufficient pollution control equipment, Xinhua news agency reported.
Checks were carried out at thousands of companies at 28 cities in and around the Beijing-Tianjin-Hebei region.
Air pollution in Chinese cities is notoriously bad.
The findings appear to confirm suspicions that companies ignore strict environmental protection policies and that officials do not enforce them, correspondents say.
Inspections found that more than 13,000 companies had failed to meet environmental standards, the ministry of environmental protection said in a statement.
The ministry ordered local officials to investigate the firms and rectify the problems, the statement said.
China's government is under pressure to do more to address the sources of smog, including reducing reliance on coal-fired power plants, the country's primary source of electricity.
Earlier this year, Beijing's mayor said a new team of environmental police would try to reduce smog levels by tackling local sources of air pollution, including open-air barbecues and dusty roads.
The mayor also promised to reduce coal consumption by 30% this year.