Postal Savings Bank of China Co is seeking as much as $8.1 billion in a Hong Kong initial public offering starting on Wednesday, which could become the world's biggest share sale this year.

The Beijing-based bank is offering 12.1 billion shares at HK$4.68 ($0.6) to HK$5.18 apiece, the bank said on Tuesday evening. Cornerstone investors, who commit to hold their shares for six months, agreed to buy about $5.9 billion of stock, accounting for 76 percent of the offering at the midpoint of the marketed range.

The price range values Postal Savings Bank at about 1 to 1.1 times its net assets at the end of March this year, a person with knowledge of the matter said, asking not to be identified as the information is private. Mainland lenders listed in Hong Kong trade at a median 0.88 times their latest book value, data compiled by Bloomberg show.

"Even though there are some positives for Postal Savings Bank, the valuation is not attractive enough," Edmond Law, an analyst at UOB Kay Hian (Hong Kong) Ltd, said. Investors may prefer the shares of other listed State-owned banks due to their more attractive price-to-book ratios, Law said.

A Hong Kong-based external spokesman for Postal Savings Bank declined to comment on the valuation.

Even at the bottom end of the price range, Postal Savings Bank's first-time share sale would be the largest globally this year, surpassing the $3 billion offering from Danish utility Dong Energy A/S in June, according to data compiled by Bloomberg. The IPO is set to be the largest globally since e-commerce billionaire Jack Ma's Alibaba Group Holding Ltd priced its $25 billion New York share sale in September 2014, the data show.

Postal Savings Bank reported an 11 percent increase in first-quarter profit as it pared provisions for bad loans, according to pre-listing documents filed with Hong Kong's stock exchange. The bank will use the proceeds to strengthen its capital base to support ongoing business growth, Tuesday's terms show.

The lender's capital adequacy ratio stood at 10.26 percent at the end of March, with a core tier-1 ratio of 8.35 percent. The China Banking Regulatory Commission requires non-systemically important banks such as Postal Savings Bank to have a minimum capital level of 10.5 percent by the end of 2018, with a core ratio at least 7.5 percent.

State-owned China Shipbuilding Industry Corp will invest about $2.2 billion in the offering, while Shanghai International Port Group Co agreed to purchase around $2.1 billion of stock, according to the terms. HNA Group Co will buy $1 billion of shares.