An electric quantity detection method includes the following steps: the initial voltage value of a power supply unit is measured; according to the measured initial voltage value, the initial surplus electric quantity value of the power supply unit is obtained through querying the voltage value-surplus electric quantity value correspondence table in a data storage module; charge/discharge electricquantity value is obtained through calculating the charge/discharge current passes through the power supply unit; according to the initial surplus electric quantity value and the charge/discharge electric quantity value, the current surplus electric quantity value of the power supply unit is calculated. Through querying the voltage-electric quantity correspondence table after measuring voltage todetermine the electric quantity value at integral starting time, the method avoids the error generated due to the electric quantity value change at integral starting time caused by self discharge, thereby increasing electric quantity detection precision. Meanwhile, the invention also provides an electric quantity detection system and an electronic device which adopt the method.