Islanding detection is a very essential function for a photovoltaic grid-connected system. Islanding phenomenons not only damage to equipment, but also endanger to the safety of the maintenance personnel. Passive islanding detection methods that use alone have considerable non-detection zone (NDZ); active islanding detection methods that use alone are that join disturbance to system, but the disturbance will cause system instability . In this paper, an over/under voltage and over/under frequency detection method combines with an improved active power perturbation detection method are proposed. The photovoltaic system can be reduced non-detection zone (NDZ) with improved the power quality and reduced the total harmonic distortion (THD) through this method.