The invention discloses a multi-side voltage reactive power coordination optimal control system for a transformer substation. The multi-side voltage reactive power coordination optimal control system for the transformer substation comprises a magnetically controlled reactor (MCR) at a medium voltage side, an MCR-type static var compensator (MSVC) at a low-voltage side, a medium-voltage bus, a low-voltage bus and a voltage and current mutual inductor, wherein the medium-voltage bus, the low-voltage bus and the low-voltage side are configured with capacitor groups respectively provided with a virtual air gap controllable reactor to provide inductive reactive power compensation; the capacitor group is configured on the low-voltage side to realize the continuous regulation from inductive reactive power to capacitive reactive power; the transformer substation is subjected to multi-side voltage reactive power coordination optimal integrated control; the coordination optimal control is carried out by the MCR at the medium voltage side and the MSVC at the low-voltage side. According to the virtual air gap controllable reactor of the multi-side voltage reactive power coordination optimal control system for the transformer substation, which is disclosed by the invention, the adjustable volume of the controllable reactor can be realized.