Was asking some people here and answer were mostly "measure theory". I also google and see this guide:
"According to J. Michael Steele, professor of stochastic calculus for the world-renowned Wharton School of Business, the minimum prerequisites for his class are probability theory, multivariate calculus, and linear algebra, the last two of which are senior-level, or graduate-level classes. He goes on to say that success in learning the subject also requires a high level of comfort with real analysis (uniform continuity, Cauchy's convergence criterion, integrability, and calculations in inner product spaces) and measure theory"
At my school, "Advanced calculus" and "Real variables" cover those topic that Professor J. Michael Steele mentioned about. I am not sure if you need to learn everything in "Advanced calculus" and "Real variables" in order to understand Stochastic Calculus throughout. However, I guess that a strong math background will never hurt.
Mathematical Analysis, 2nd edition, by T. M. Apostol is used in Advanced calculus. Topics are from chapter 1-14.
RealAnalysis, 4th edition, by H.L.Royden is used in Real variables. Topics are from page 1-130, 253-281 and 308-424. However, a member here told me it is a little bit overkill for MFE. I still want to take this course if I can though.