The sales slump is the latest sign from GSK(GSK) that the probe is hurting its business. The firm said it continues to co-operate with authorities and remains "fully committed" to supplying products to China.
Sales in China are just a fraction of overall revenue but the country represents one of the company's fastest growing markets.
Glaxo has invested heavily in China, setting up clinical centers and research facilities in Tianjin and Shanghai.
Corruption is thought to be endemic in wide swaths of Chinese industry and is perceived by many as a cost of doing business in there. China however, appears determined to clean up its healthcare sector, and is investigating price-setting practices at 60 pharmaceutical companies.