Google says it will price its latest stock sale at $295 (£162.55) a share, more than three times the price of its initial public offering last year.
Google's shares first came on the market more than a year ago
The sale of more than 14 million shares, expected on Thursday, is set to raise $4.18bn (£2.3bn).
The proceeds will partly be used to help fund possible acquisitions.
Google went public in August 2004 with shares priced at $85. They closed at $303 on Wednesday, down 2.8%, before the new stock sale was announced.
"The principal purpose of this offering is to obtain additional capital," Google said in a filing last month with the US Securities and Exchange Commission.
However, it has given no firm details of how it plans to use the money, leading to widespread speculation in the technology industry.
Commentators have suggested that California-based Google may use the cash to gain a stronger foothold in China, following a move by internet rival Yahoo to take a 40% stake in Chinese auction website Alibaba.com.
Google is the number two search engine in China with 23% of the market, trailing local rival Baidu.com, which has 37%.
Google courted controversy when it hired former Microsoft executive Kai-Fu Lee to head its Chinese research and development facility.
Microsoft went to court in the US to try to block the move, but a judge in Seattle allowed Kai-Fu Lee to join Google with some limitations.
In other moves, Google has been steadily expanding its operations beyond its core internet search sites, to include free e-mail, web logs and online comparison shopping.