In this paper, a time-dependent model of directionally solidified Al-CuAl2 eutectic system is numerically calculated according to the theory of lamellar eutectic growth. The computed minimal and maxiamal interlamellar spacings correspond well to the experimental results,the lamellar spacing increases due to melt flow in solidification, and such structure is unstable as the spacing exceeds the maximum. Moreover, the lower the growing rate, the greater the effect of fluid flow.