First, the default maps are in Online mode. The Offline mode maps have very little detail across the globe, but the Online maps go down to street/block level. At least in the U.S. - there is less detail across the full globe, which I believe is what you're referring to.
Second, Tableau supports using other Web Map Servers that follow the WMS protocol. See Data -> Background Maps -> WMS Servers... and the help for more details. There are some public and free WMS servers on the internet, though I don't know that Google Maps is a provider.
Thanks for your prompt reply. Google map should be a WMS provider, but I can't find its url. Is there any resources about wms that we can connect to use the online map function?
WMS servers are often set up internally by companies that have custom maps that they want to make available to the entire company. Finding public WMS servers that have the exact detail/aesthetic that you are looking for can be a little difficult. There are some links to public WMS servers on the WMS Wikipedia Page.