The transmission equation of femtosecond soliton is given considering the effects of fourth order dispersion,and its effects on pulse shape and pulse spectra are analyzed numerically.The conclusion show that the pulse width is expanded and the side spectra on caused by fourth order dispersion were obtained.The method to compensate the pulse phase by the unusual second order dispersion and cosine phase modulation have been given.The result that pulse phase can be kept smoothness in 220 ps time sect and the pulse width can be kept the original size.