We demonstrated an effective method of Ar surface plasma treatment (SPT) to improve the resistive switching (RS) uniformity of HfO2-(x) based resistive random access memory (RRAM) device. More importantly, the operation of multilevel RRAM and low power consumption can be further achieved by reliable RS, which enabled to obtain five distinguishable low resistance states and can be operated with a low power consumption of similar to 30 pJ. The results of atomic force microscope and X-ray photoelectron spectroscopy analysis confirmed that the Ar SPT induced more oxygen defects and higher roughness on the surface, which decreased the migration barrier of oxygen migration and some tips below the electrode. Eventually, a moderate forming process and local electric-field enhancement around these tips were obtained, accounting for the improvement of RS uniformity. This method could be promising to develop RRAM with high uniformity for the low-power multilevel memory applications.