Mr Humphrey and his wife, Yu Yingzeng, were sentenced in August last year for illegally obtaining private information on Chinese nationals.
He received a two-and-a-half year sentence, while Ms Yu was sentenced to two years in prison.
GlaxoSmithKline (GSK) China, which was fined $490m in China for bribery, hired Mr Humphrey in 2013.
The pair were hired by GSK to investigate the source of a sex tape of the company's China boss, shortly before the firm became the subject of the bribery allegations.
They admitted buying information, but said they did not realise it was illegal to do so.
Release in the coming days will reduce his sentence by seven months. Time spent in custody before his trial was taken off his sentence.
Once released, he is likely to be deported, as the court originally stipulated.
Ms Yu will be released in the coming weeks, according to the Reuters news agency.
The original verdict did not make a link between Mr Humphrey's investigations and the allegations against the pharmaceutical giant GSK.
Those allegations first emerged in an email in January 2013 from an anonymous and self-styled whistleblower to the company.
The email alleged that GSK's sales teams targeted influential doctors with expensive gifts and cash to win business.
It was sent to GSK's London-based chief executive, Andrew Witty, and made accusations against GSK China's general manager Mark Reilly.
Later, a sex tape featuring Mr Reilly and his girlfriend was also sent.
GSK declined to comment.
British company investigator Peter Humphrey is to be released early from a Chinese jail, according to reports.