The present invention discloses a transient voltage dq resolving method for grid voltage drop detection, and relates to the field of voltage drop detection. A single-phase system voltage signal is virtualized, and a space vector superposition principle is used, to improve a grid voltage drop detection algorithm and a transient voltage dq resolving method that are currently commonly used, so that adelay time of the foregoing method is further shortened. In a power frequency condition, compared with 3.33 ms in the transient voltage dq resolving method, a theoretical delay time of the present invention is 0.83 ms, and an actual delay time is only 0.9375 ms, so that instantaneity is substantially improved. According to the method, a multiphase system is virtualized by using a single-phase voltage signal, the quantity of alternating current voltage sensors is reduced, and costs are lowered. The present invention is simple in algorithm, practical, and good in dynamic performance, and has astrong engineering practice function.