To solve voltage sag caused by mutation in reactive power on transmission lines, this paper proposed a real-time detection method for single-phase instantaneous reactive current based on dq transformation. Firstly, a virtual orthogonal signal construction algorithm based on trigonometric transformation is introduced to transform the single-phase current in stationary dq coordinate system to synchronous rotating coordinate system quickly and accurately; then, in order to filter the harmonics and noise components in the current signal, an enhanced moving average filter (EMAF) is designed and the moving length is optimized; finally, this paper conducted simulation experiment in MATLAB/Simulink and built single-phase chain STATCOM experimental device for verification. The results show that the proposed detection method can track the variation of instantaneous reactive current in single-phase distribution network, realize the high-performance control of single-phase chain STATCOM, and ensure the stable operation of the distribution network before and after the mutation in reactive power, which verifies the effectiveness and advancedness of the proposed method.