With the rapid development and application of new energy and power electronics technologies, the current grid voltage fluctuations and flicker problems are becoming more and more seriously. Aiming at the accurate detection and evaluation of voltage flicker parameters, a method for detecting voltage flicker envelope parameters based on improved energy operator and six-term cosine window three-spectral line interpolation is proposed. Compared with traditional energy operator, this method does not need square root operation, and has faster calculation speed and better real-time performance. The accuracy and stability of the system in the case of fundamental wave frequency fluctuation and various harmonics are improved by the flicker correction coefficient. A detection platform based on PXI + LabVIEW architecture is built and tested. The simulation and experimental results show that the detection algorithm is more stable and accurate than the traditional energy operator when it contains single amplitude modulation, multi-frequency amplitude modulation, fundamental frequency fluctuation, harmonic, inter-harmonic and white noise interference respectively. And the real-time online detection and accurate analysis of voltage flicker parameters can effectively be realized. This work is supported by National Natural Science Foundation of China (No. 51777061) and Science and Technology of Guizhou Power Grid (No. GZKJXM20171048).