The researches on the grain refinement by applied pulsed magnetic field (PMF) during solidification have received much attention in recent years and lots of positive experimental results indicate that it is a potential method for controlling solidification process. Various grain refinement mechanisms under PMF are proposed and most of them are considered to be relevant to the convection of melt driven by the electromagnetic force. An obvious fact is that the forced convection caused by PMF is strongly limited by the shape of the melt. However, most of previous studies were focused on the cylindrical samples rather than rectangular ones, and actually the later one was widely used in industry. The aim of this work is to investigate the influence of PMF on the grain refinement of K4169 superalloy rectangular samples with various aspect ratios. Grain refinement of K4169 superalloy under PMF was experimentally investigated in the rectangular samples with the aspect ratios of 1.0, 2.0, 4.5 and 5.5 on the transverse section. In order to study the influence of aspect ratio on the forced convection, the distributions of the electromagnetic field, electromagnetic force and melt flow caused by PMF were numerically simulated by finite element software ANSYS. The experimental results show that the grains of the K4169 rectangular samples are coarse equaxied grains without PMF and the grain size slightly decreases with the increase of aspect ratio . Under the PMF with same excitation voltage and frequency, the grains are refined remarkably in the sample with the aspect ratio of 1.0. As the aspect ratio is increased, the grain refinement effect can still be observed but not such obvious. The numerical simulation results indicate that the periodic pushing-pulling electromagnetic force is induced by the PMF, which drives the melt to vibrate and flow circularly. Under the same PMF, the electromagnetic force and fluid rate decreases with the increase of aspect ratio. When the aspect ratio increases from 1.0 to 5.5, the average electromagnetic force and fluid rate in the melt is reduced to 40% and 60%, respectively. The strongest fluid flow and vibration occur in the sample with section aspect ratio 1.0 in the present experiment, which is beneficial for grain refinement due to detachment of the solidified nuclei from mould wall and the break of dendrite arms from dendrite trunks.