The high order harmonic emission from thin plasma foils irradiated by two circular polarized, counter propagating laser pulses with their electrical vectors rotating in different direction was investigated via 1\|D particle in cell (PIC) simulations, More than 200 harmonics can be generated with the laser intensity of 10 21 W/cm 2. When the duration of laser gets shorter, the frequencies of harmonics were severely modulated due to the Doppler shift caused by the movement of the plasma boundary when the foil is being compressed. The Doppler shift can be estimated by the simulation results, and this effect can also be reduced or modified by introducing frequency chirping to the pump pulse.