In order to solve the operation conflict between islanding protection and low-voltage-ride-through of grid-connected photovoltaic system, a simultaneous detection method for photovoltaic islanding and low-voltage-ride-through based on harmonic characteristics is proposed. According to the differences of the system harmonic voltages among three operating states, i.e. grid-connected operation, islanding operation and voltage transient perturbation, the influence of spectrum leakage on harmonic detection is quantitatively analyzed by Fourier series. After the influence of the background harmonic in main grid is considered, the setting formula of the harmonic voltage threshold is derived. Based on that the simultaneous detection implementation scheme and its algorithm flow are given, which are verified on simulation software platform. The harmonic characteristic curve extracted from the data of “Islanding Operation Test of Xiexin-Bianzhao Photovoltaic Power Station Carried with Bianzhao Transformer” further verifies that the proposed harmonic voltage threshold can identify the islanding phenomenon. Both theoretical analysis and simulation experiments show that the proposed harmonic voltage threshold can be used as the basis to distinguish the islanding phenomenon from the voltage transient perturbation phenomenon. And the proposed simultaneous detection method based on harmonic characteristics can effectively coordinate the two functions of grid-connected photovoltaic system, i.e. islanding protection and low-voltage-ride-through. This work is supported by the National Key Research and Development Program of China (No. 2017YFB0902002).