Clarifying the nonradiative carrier recombination centers in photovoltaic materials is of both fundamental interest and technological significance. For a defect with considerable concentration in the semiconductor material, whether it is an effective recombination center and limits the photovoltaic efficiency should be quantitatively judged by its carrier-capture cross sections, which are difficult to determine in both experiment and theory. The direct experimental measurement is often difficult, especially to identify the responsible defects, highlighting the importance of using an ab initio approach to clarify the situation. Here we use a recently developed method to effectively calculate the carrier-capture cross sections of deep-levels defects in Cu2ZnSnS4. All the physical parameters are quantitatively determined based on the state-of-the-art ab initio calculations, which makes the result more accurate and reliable. Furthermore, the microscopic mechanics of recombination and the favored growth conditions for high-efficiency Cu2ZnSnS4 solar cells are also revealed. Similar calculations can be used for studying the influence of various defects on the photovoltaic performance of other solar cell materials, shedding light on the design of high-efficiency solar cells.