STATCOM can compensate the reactive power change that causes the voltage sag issues of distribution grid with long feeders. In this paper, a novel instantaneous reactive current detection method is developed, overcoming the limitation of conventional methods in detection speed, computation burden and noise/harmonic susceptibility. An improved virtual orthogonal signal generation method based on mathematical deduction is introduced first, remarkably improving the immediacy and precision of detection in dq frame. Then an enhanced moving average filter is utilized in cascade and its design principle is proposed, sufficiently eliminating the effects of noise and harmonics while preserving fast dynamic speed. Finally, high-performance control of single-phase STATCOM is realized utilizing the proposed method. Experiments reveal that the proposed detection scheme is characterized by its fast speed, high precision as well as noise/harmonics immunity.