Topological superconductivity makes it possible for fault-tolerant quantum computations. Fe-based superconductors are a rich material family of intermetallic compounds for high-temperature superconductivity. The interplay between these two exciting fields creates an emergent new field, marked by recent discoveries of the superconducting Dirac surface state and Majorana bound states at the vortex cores in iron-based superconductors Fe_1+yTe_1−xSe_x. However, the direct electronic transport consequence is not clear out of these topological superconducting states. We are left wondering how to operate these novel excitations. We solve the problem by thermally drive ordinary superconducting vortices around these Majorana zero modes and search for a transverse electrical response. This scheme is the Nernst effect in the superconducting state. A finite Nernst signal is observed at zero fields in the superconducting states of Fe_1+yTe_1−xSe_x. This anomalous Nernst signal shows field-symmetric dependence on the external magnetic field. Our experiments provide the first evidence of broken time-reversal symmetry in topological superconductors and point to a new method to engage Majorana zero mode excitations in transport properties.