Several techniques of femtosecond pulse spectrum broadening in the chirped pulse amplifier of femtosecond laser are reported, and a few spectrum broadening elements are used. The results show that the spectrum of femtosecond pulse laser can be broadened effectively by using the laser spectrum broadening elements, and at some extent the spectrum gain narrowing effect of laser pulse can be effectively suppressed in the femtosecond laser amplifier. The seed pulse output of 28 fs is obtained from the oscillator in the femtosecond amplifying system. After amplification, an ultrashort ultra intensity laser pulse with the repetition rate 10 Hz, peak power 1.6 TW, pulsewidth 37.5 fs is achieved.