The effects of soil bulk density (SBD) on both leaf water use efficiency (WUEl) and plant water use efficiency (WUEp) of maize (Zea mays L.) under different water regimes were studied. Split-root experiments were conducted under both low (-0.86 MPa) and high (-0.17 MPa) soil matrix potential conditions. Four types of soil treatments were imposed: low density (both compartments packed to 1.20 g/cm³), middle density (both compartments packed to 1.33 g/cm³), high density (both compartments packed to 1.45 g/cm³), and mix density (one compartment packed to 1.20, the other to 1.45 g/cm³). The results showed that under drought condition (-0.86 MPa), WUEl increased and WUEp decreased with increasing SBD, whereas under wetting condition (-0.17 MPa), both WUEl and WUEp increase with increasing SBD. The correlation analysis showed that WUEl had positive correlations with soil mechanical resistance (Rs) under both drought and wetting condition. There were positive correlations between WUEp and Rs under wetting condition, but significant negative correlation under drought stress. Under both drought and wetting condition, the WUEl and WUEp of plant grown in mixed density soil maintain higher level. It is suggested that high SBD and low matrix potential influence water use efficiency (WUE) independently, but the effects of low matrix potential is more profound, and there is also an interactive effect of them upon WUE. Meanwhile the spatial change of SBD is beneficial for WUE improvement.