According to the theory of chirped pulse amplification (CPA), we analyze numerically the gain characteristic of laser pulses in multi-pass amplifier. A CPA Ti:sapphire laser with 10-pass scheme is designed and built. The experimental result shows that the contrast ratio of the femtosecond pulse is obviously enhanced by two orders from 10-5 to 10-7 under a pump flux of 1.6 J/cm2. This work demonstrates that the contrast ratio of the amplified femtosecond laser pulse could be effectively improved by multi-pass preamplifier.