A digital integration method breaker dielectric loss angle tangent value online monitoring system and a method thereof are disclosed. The system comprises a monitoring host, a sampling calculating CPU, an A/D conversion unit, a phase-locked loop circuit, an isolating amplification circuit, a low pass filter circuit, a voltage transformer, the A/D conversion unit, the isolating amplification circuit, the low pass filter circuit, a current transformer, a breaker, a keyboard, a display unit, a CAN communication unit, a memory, an alarm unit, a programming interface, a clock unit, a reset unit, an RS-485communication unit and a central control chamber. An operation process only aims at a fundamental wave, which means that filtering is performed on the fundamental wave, and an influence of a harmonic wave on an operation result is eliminated. The technology possesses advantages that the structure is simple; measurement precision is high; stability is good; real-time performance is good too; and anti-interference capability is high and so on.