The so_called “phase detune”, caused by nonlinear effects which increase the wavelength of the wake wave, when its amplitude exceeds some limitation, is the m ain saturation mechanism of wake waves in multiple pulse laser wake_field accele rator(MP_LWFA). The MP_LWFA with the optimized spacing between the pulses equal to the wavelength of the wake wave, is studied by PIC simulation in this paper. Our simulation indicates that the detuning length and thus the amplitude of the wake wave increases as our expected. Furthermore, the stronger forward Raman sca ttering is stimulated, which plays an important role in stopping the increase of wake waves.