Objective   To evaluate the biomechanical efficiency of single-level unilateral pedicle screw fixation on the instrumented lumbar segment and adjacent segments. Methods  Six fresh-frozen human cadaveric lumbar spines of simulated instability at L4~5 were conducted to a serial of biomechanical tests including flexion, extension, lateral bending and axial rotation. The range of motion in each specimen was tested in sequence according to such four groups: intact in group A, L4-5 segmental instability in group B, unilateral pedicle screw fixation combined with single cage in group C and bilateral pedicle screw fixation combined with single cage in group D.  Results  Apart from the ROM of left lateral bending, there was no significant difference of ROM at L4-5 between group C and group D (P>0.05). Apart from the ROM of left and right lateral bending, there was no significant difference of ROM at L3-4 and L5-S1 between group C and group D (P>0.05). Conclusions  All tested single-level unilateral pedicle screw fixation can achieve similar initial biomechanical stability and cause less lateral bending motion at adjacent segments in comparison with bilateral pedicle screw fixation.