Generally, it is AdSense policy to not allow Google ads to open in a new window. However, in a few markets, including China, AdSense has found that opening ads in a new window better aligns with expected internet behavior and provides a more consistent, intuitive user experience.
Keeping in mind the importance of the user experience, AdSense has enabled Google ads to open in new windows in locations where this is expected browsing behavior. This functionality is determined by the IP of the user, and is not an option you can select your AdSense account.
AdSense also ask that you don’t modify the AdSense ad code or alter the result of clicking on an ad on your website, as such behavior violates AdSense program policies.