The radio approach for detecting the ultra-high energy cosmic neutrinos has become a mature field. The signals in radio detection are originated from the Cherenkov radiation of particle showers. We present a calculation method of Cherenkov pulses based on finite-difference time-domain (FDTD) method. Our method provides a straightforward way of calculating the near field radiation, which would be important for ultra high energy particle showers, especailly the electromagnetic showers induced by the high energy leptons produced in the neutrino charge current interactions.