Nonreciprocal charge transport phenomena are widely studied in two-dimensional superconductors, which demonstrates unidirectional anisotropy behaviors as a result of symmetry breaking. Here, we report a strongly nonreciprocal transport phenomenon in superconducting CsV3Sb5 thin flakes. The second harmonic voltage, mainly originates from the rectification effect of vortex motion, is unambiguously developed with in-plane and out-of-plane magnetic fields, and its magnitude is comparable to that in noncentrosymmetric superconductors. The second harmonic magnetoresistance splits into several peaks and some of them reverse their sign by ramping the magnetic field or current within the superconducting transition. The vortex dynamics for the specifically nonreciprocal transport phenomenon is dominated by an unconventional superconductivity pairing symmetry, providing a promising path to explore the possible topological superconductivity in CsV3Sb5.