Liquid concentration distribution is seriously affected by an abruptly changing pulling velocity under directional solidification. Theoretical and numerical investigations indicate that at the pulling velocity jumping from V 0 to V, the solidification system does not achieve the pulling velocity V immediately, and it goes through a non-steady-state transition zone. As the pulling velocity abruptly increases (V/V 0 > 1), interface liquid concentration firstly increases to the maximum and then decreases to the steady-state value. The magnitude of interface liquid concentration at the beginning increases with V/V 0 , the initial pulling velocity V 0 and the temperature gradient G L in the liquid. At the same time, solute diffusion length reduces with V/V 0 and G L . In contrast, the minimum of interface liquid concentration falls with V/V 0 at the pulling velocity decreasing abruptly. As the interface liquid concentration enriched at V/V 0 > 1 is more than the value required for the planar interface to keep stable, the solid/liquid interface may become unstable. The analytical results are in agreement with the numerical calculation results of Al-2%Cu alloy. directional solidification, abruptly changing velocity, concentration distribution, Al-Cu alloy Many important solidification models to predict microstructure formation, such as constitutional supercooling criterion (CSC), Mullins-Sekerka stability theory and Jackson-Hunt model in eutectic alloy etc., were developed using directional solidification processing, since the pulling velocity and temperature gradient in directional solidification can be easily controlled and adjusted respectively [1][2][3] . In fluid mechanics, directional solidification processing helps us to well understand interface pattern formation and evolution far away equilibrium condition. At the same time, some new materials and products with high-performance, for example of magnetic materials, quasi-crystal and single crystal superalloy turbine etc., can be manufactured directly during directional solidification [4][5][6] .
