The invention provides an automatic deionized water supply device for a cooling system of an SFC (static frequency converter). The automatic deionized water supply device comprises a support assembly, a deionized water preparation unit, a deionized water supplementing unit and a control unit, wherein the deionized water preparation unit comprises a deionizing pump, a resin deionizing bottle, a conductivity measuring sensor, a valve A, a circulating pipe, a water storage tank and a data display instrument with upper limit and lower limit alarming functions, the water storage tank, the deionizing pump and the resin deionizing bottle are communicated sequentially through the circulating pipe to constitute a deionized water preparation loop, the data display instrument is electrically connected with the conductivity measuring sensor which is mounted on the water storage tank, and the valve A is mounted on the circulating pipe; the deionized water supplementing unit comprises a submersible pump, a valve B, a check valve and a supply pipe, and the supply pipe, the submersible pump and the water storage tank are communicated sequentially. The automatic deionized water supply device has automatic deionized water preparation and supplementation functions, trouble in manual water supplementation is avoided, the working efficiency is increased, and the deionized water preparation process is simple, practical and efficient.