The evolution equations for the parameters of the optical Gaussian-shaped pulse in optical fibers are derived by means of variational method from the MNLS equation. Then three restricted relations between amplitude and width,frequency and chirp,width and chirp are derived. Analytical solution about the width of pulse evolves on the propagation distance is obtained. The influence of the fifth-order nonlinearity on the propagation properties of Gaussian-shaped pulses in optical fibers is discussed. And the profiles of the evolution for the fifth-order nonlinearity(r),chirp(b),initial chirp(b₀),distance(Z)of the frequency band width(a)are drawn.