The effect of a high magnetic field (up to 12T) on the lamellar eutectic spacing changes was investigated in the diffusion liquid Al/solid Cu. It was found that the lamellar eutectic alloy is formed in the diffusion process and its spacing has the non-monotonic relationship with the magnetic field intensity. With the increase of magnetic field intensity, the lamellar eutectic spacing decreases rapidly. When the magnetic field intensity B exceeds 4T, the lamellar eutectic spacing begins to increase until 8T, and then, decrease again. This phenomenon could be attributed to the effects of high magnetic fields suppressing nature convection and inducing thermo-electromagnetic convection in the liquid Al.