This study numerically investigates the effects of changing vaned diffuser angles and vaneless diffuser widths of a twostage centrifugal refrigerant compressor on the reduction of surge flow rate and the improvement of part-load performance. The first stage has a low-solidity vaned diffuser, while the second stage has a vaneless diffuser. The analysis results show that adjusting the angle of low-solidity vaned diffuser in the first stage can lower the surge flow rate and keeps high efficiencies at low flow rates. The inlet guide vane of the first stage can also lower the surge flow rate; however, their simultaneous actuation cannot lower the surge flow rate further. Reducing the vaneless diffuser width of the second stage can lower the surge flow rate. Meanwhile, simultaneously adjusting the individual inlet guide vanes of the second stage can also effectively lower the surge flow rate. If the diffuser width of the first stage is reduced while the low-solidity vaned diffuser blade angle is fixed, the surge flow rate will be lowered obviously as the inlet guide vane is adjusted. However, reduction in diffuser width decreases the efficiency at low flow rate. Controlling the inlet guide vane and diffuser width well can lower the surge flow rate to 30% of the design flow.