The invention provides an in situ monitoring method and system for state-of-charge of a positive or negative electrolyte for a vanadium battery. The method comprises the following steps: respectively measuring oxidation-reduction potential (ORP) of positive or negative electrolyte for the battery respectively, and obtaining the state-of-charge (SOC) of the positive or negative electrolyte for the battery; respectively building relational expressions between ORP and ln (1/SOC-1) of the positive or negative electrolyte for the battery in the charging and discharging processes through a curve fitting method according to the ORP and the SOC; and obtaining the SOC of the positive electrolyte for the battery or the SOC of the negative electrolyte through the relational expressions between the ORP and the ln (1/SOC-1) in real time according to the ORP of the positive electrolyte for the battery or the ORP of the negative electrolyte, which is obtained through real-time measurement, so as to obtain relatively accurate state-of-charge. In situ monitoring of the state-of-charge of the positive or negative electrolyte for the vanadium battery is achieved through relatively simple and convenient means.