The invention provides a composite electrical fire monitoring detector and a monitoring method thereof. The monitoring detector comprises an input voltage detector, a residual current transformer, a first current transformer, a second current transformer, a third current transformer, a fourth current transformer, a temperature sensor, a first resistance-capacitance filter network, a second resistance-capacitance filter network, a third resistance-capacitance filter network, a 4th resistance-capacitance filter network, a 5th resistance-capacitance filter network, a 6th resistance-capacitance filter network, a two-channel analog front end, a four-channel analog front end, an amplification filtering unit, a microprocessor, an output control and LED indicating unit, a liquid crystal display unit, an NB-IOT module unit, a storage unit, an RS485 communication interface unit and an AC / DC power conversion unit. The monitoring method is realized by using the monitoring detector. Therefore, false alarming caused by the common mode interference signal is reduced; the accuracy of the residual current and three-phase current effective value measurement is improved; and the high measurement accuracy of the high-low-end signals is ensured while the measurement range is extended.