Mathematical finance, as you said, is professionally called the major or direction of financial engineering, which requires higher mathematics foundation. Now some schools have such a research direction. In the field of financial specialty, the strongest in China is the Graduate School of the People's Bank of China (Wudaokou, Beijing). In addition, university of international business and economics Institute of Finance and Institute of International Economics and Trade have graduate students in this direction. Relatively speaking, there are not many graduates in this major so far, and the employment prospects are hard to say. If you have a good mathematical foundation, can you consider the actuarial direction? At present, there are not many people in China who can get an actuary's certificate.