X-ray pulses of a few hundred attosecond play an important role in researches of ultra-fast science On enhanced self-amplified spontaneous emission (ESASE) mode. peak current of electron beams is much higher and the gain length is much shorter, in comparison with the self-amplified spontaneous emission mode in most running free electron laser (FEL) facilities. Based on typical parameters in soft X-ray free electron laser, this paper conducts an optimized simulation on ESASE. Simulation results show that hundreds attosecond X-ray of tunable wavelength in water window band is obtained, with a peak power of more than 1GW using a typical 2.5 GeV electron beam. This paper provides a reference to our following experiment on ESASE, and also sets a basis to the ongoing optimization of experimental parameters.