The invention relates to a negative sequence voltage positive feedback-based islanding detection system. The detection system is characterized in that the detection system includes a DSP control chip, a sampling and conditioning circuit, a PWM isolation driving circuit, a digital-to-analog (DA) conversion circuit, a power source unit, a communication unit, an expansion circuit unit and a voltage and current protection circuit unit. An islanding detection method of the system includes the following steps that: the positive and negative sequence voltage components of a photovoltaic system are extracted; phase-locked loop signals are inputted; current disturbance quantity is obtained; and whether islanding occurs is judged. According to the islanding detection system and method, fast islanding protection under power grid unbalance can be realized through adopting a negative sequence voltage positive feedback method; and variable coefficients are adopted, so that blind zone-free islanding detection can be realized under the condition that current given values are different.