The invention relates to the field of battery energy storage, in particular to an SOC estimation method of a series-parallel combined type battery stack and a battery management system. SOC values ofall single batteries connected in series in each battery cluster are collected and an SOC value of each battery cluster is calculated; the SOC values of all the battery clusters are clustered to obtain clustering center point; an SOC value of the battery stack is calculated based on the clustering centers. The obtain SOC value of the battery stack can cover the full-interval change of [0, 100]; the SOC value change of the battery stack becomes smooth; the influence of the sudden change on the SOC value estimation of the battery stack in the charging and discharging process is avoided; and thusthe SOC value estimation result of the battery stack is accurate.