My fiance from China now has a B1/B2 visa, which is a tourist visa. Is it possible to wed in the States and then file for the spousal visa?
Yes, you can get married in the US while on a tourist visa. It happens all the time, and doesn't always have anything to do with immigration. For example, two Japanese tourists getting married in Hawaii. They obviously aren't eligible for an immigration benefit, but are more than welcome to enter the US as a tourist (or VWP in the case of Japan) then get married.
What you really are trying to figure out is can she come to the US as a tourist, then get married and stay in the United States as an immigrant, or if you can get married, then she returns to China, and then applies for a spousal visa to return as an immigrant.
You could definitely get married in the United States, then could return to China and you could file a spousal visa. In this case, she waits there for that whole process to work its way through.
The other scenario would be getting married in the United States as a tourist and adjusting her status while she's here from nonimmigrant to immigrant status. If the intent was premeditated to circumvent the fiance visa process, which is the visa type designed for this process, this would be illegal. However, if she was visiting as a tourist and originally intended to return to China but you spontaneously got married, and you have all the evidence to prove this intent, she could technically adjust her status. Please note, this type of case is seeing more scrutiny and we highly discourage it.
Please see our article: US State Department Will Now Deny Adjustment of Status on Tourist Visas.