Coaxial feed structures are widely used in ultra-wide band antennas . This paper modeled the characteristic of the monopole antenna feeded by coaxial line by FDTD in the time-domiain,which showes that . Firstly, it introduced the theory of the arithmetic and the particularly realization in the calculation then it described the use in the time-domain finally it analysed several characteristics of the monopole antenna. The arithmetic used in the microstrip antenna is also a quick and economical way to design the antenna.