I'm answering my own question. 6 months ago, I accepted Prometheus's answer, I really thanked him for searching information for me. But I removed the acceptance, because I afraid that people may get wrong information.
The reason I want to apply a B2 visa for my wife, because J-2 is not reasonable for us. First, my internship was only 3 months, and my wife only intended to visit me for 1 month. However, if we applied for the J-2 visa, we would have to pay the insurance for 3 months for my wife. i.e. as long as my stay. And believe me, the insurance is extremely expensive.
After searching for a while, I realized that spouse or under-21 children who intend to stay less than 6 months (some pages say only 3 months) in the US have the option to apply for a B visa. So I applied for a J-1 and my wife applied for a B2. We arranged to have the appointment at the same time, but the US Embassy in London processed our applications separately.
The funny thing was that: my J-1 visa was valid only for 5 months, but my wife's visa is valid for 1 year. Moreover, she applied for a B2 visa, and they granted her a B1/B2 visa.
However, note that a J-2 application is easier to be accepted than a B2, since the dependent also has the sponsor from the host company etc. We took a risk of applying B2 because my wife had travelled to the US alone several times before.