By calculating the momenta of a coupled set of nonlinear equations of laserpulse-plasma wave interaction in the weak relativistic approximation,the conditions for fre-quency up-shift have been found.That the energy change of the pulse due to frequency shiftis compensated by the change of plasma wave energy results in photon number conservation.Some factors that affect the frequency up-shift are studied.It is also found that the laser pulsecan be compressed when it is located near the valley of plasma density perturbation and ifsome threshold value of the plasma wave field is exceeded. By calculating the momenta of a coupled set of nonlinear equations of laserpulse-plasma wave interaction in the weak relativistic approximation, the conditions for fre-quency up-shift have been found. THAT the energy change of the pulse due to frequency shiftis compensated by the change of plasma wave energy results in photon number conservation. Some factors that affect the frequency up-shift are studied. It is also found that the laser pulse can be compressed when it is located near the valley of plasma density perturbation and ifsome threshold value of the plasma wave field is exceeded.