The directional cellular growth of SCN-0.2%Salol transparent alloy in the presence of shear flow at the liquid-solid interface is in-situ observed, and the cellular spacing adjusting mechanism is carefully investigated. It is found that the cellular array deflects forward the incoming flow direction, and the stable cellular spacing decreases with the increase of the flow rate. This is due mainly to the reducing destabilization wavelength caused by shear flow. Instead of the double symmetric splitting mode under static condition, the splitting mechanism of cellular growth exhibits multiplicity, which is characterized mainly by (i) multi-splitting and asymmetric splitting, and (ii) secondary branches appearing on the upstream side whose growth direction subsequently shifts to ward the direction paralleled to the trunk to form new cells. Meanwhile, the selected mechanism transits from the weak cells eliminated by the relatively strong ones on both sides during static solidification to the growth of weak cells on the downstream side suppressed by the stronger cells at the upstream side when shear flow is applied.