Potassium dihydrogen phosphate (KDP) single crystals doped with molybdate (MoO4 (2-)) were grown via the conventional temperature cooling and rapid growth methods, respectively. MoO4 (2-) made KDP crystals tapering for conventional temperature cooling method. When KDP crystals were grown by rapid growth method, MoO4 (2-) could induce liquid inclusions and simultaneous crystals. The measurement on growth rates indicated that MoO4 (2-) broadened the dead zone and decreased the growth rate of (100) face of KDP crystals. The growth kinetic analysis in terms of two-dimensional nucleus and screw dislocation models implied that the energetic parameter gamma/kT decreased with an increase of MoO4 (2-) concentration. The influence of MoO4 (2-) growth steps on (100) face of KDP crystal was observed through ex situ AFM technique. It gave evidence that MoO4 (2-) could postpone the step bunching and make the step edge curving and knaggy to reduce the edge free energy, which was in agreement with the growth kinetics calculations. Additionally, the poisoned mechanism of MoO4 (2-) and Fe3+ on step morphologies was detailed contrasted. The interaction process was discussed according to electro negativity analysis, which indicated MoO4 (2-) (actually were HMoO4 (-) and H2MoO4) could be absorbed onto (100) face through charge-assisted hydrogen bonds and caused more Mo element distributed in prismatic sector.