The transaction is in line to be China's second-largest flotation on the Hong Kong Stock Exchange, following the $9.2bn (£5.2bn) listing of the China Construction Bank last year.
It is understood that Freshfields won the work from the bank after a competitive tender process. It is the first time the firm has advised it. Hong Kong corporate partner Kay Ian Ng is leading the team.
Sullivan & Cromwell is acting for the Bank of China on US law aspects of the deal, while Shearman & Sterling has secured an advisory role for underwriters Goldman Sachs and UBS, with capital markets partner Leiming Chen taking the lead. Allen & Overy is also representing the underwriters on Hong Kong law.
King & Wood and Jun He Law Offices are understood to be responsible for the local law aspects of the listing.
One source said: "The transaction is still in its very early stages. It may list as soon as [this week], but I don't think so - it has to be approved by the China Securities Regulatory Commission."
Bank of China will be the second of the country's top four banks to list on the Hong Kong Stock Exchange.