BiMn3Cr4O12 shows an unusual joint multiferroicity, which facilitates the coexistence of considerable ferroelectric polarization and remarkable magnetoelectric coupling in a single-phase multiferroic material. Based on first-principles calculations, we investigate the two different types of ferroelectric phase transitions in the BiMn3Cr4O12 material. Our results show that the first ferroelectric phase transition is driven by soft mode and leads BiMn3Cr4O12 into the Cm space group. The predicted ferroelectric polarization in single crystal is about similar to 9.8 mu C/cm(2). With the emergence of spin order on both Mn and Cr sublattices, it is the polar Cm structure that triggers the exchange striction mechanism and therefore results in a large type-II multiferroicity (similar to 1.1 mu C/cm(2)). In addition, the intrinsic direction of the spin-driven ferroelectric polarization is always opposite to that of the existing Cm phase structure. Our results imply a feasible strategy in searching/designing novel type-II multiferroics with large ferroelectric polarization.