A dual-arm phase object (PO) Z-scan technique is proposed based on PO Z-scan technique, in which an identical optical path is added into the PO Z-scan setup. Compared with the conventional Z-scan technique, PO Z-scan technique can distinguish the third-order nonlinear refraction from the transient thermally induced nonlinearity and realize the single pulse measurement, and the system sensitivity is greater than Z-scan technique. However, previous methods cannot extract the dilute solute nonlinearities from solution well, the dual-arm PO Zscan technique can greatly improve the measurement signal-to-noise ratio, and the dilute solute nonlinearities from solution can be measured accurately.