In order to reduce the impact of dead zone fault on transformer from the 220 kV main transformer mid-voltage side， the various type fault characteristics of main transformer mid-voltage side dead zone fault in 220 kV substation are analyzed， and the microprocessor-based protection circuit for transformer is studied. Finally， it concludes that the mid-voltage side dead zone protection should be added to the existing backup protection. After this improvement， the protection act conditions according to the analysis from the mid-voltage side dead zone fault feature and the protection action time according to the rated breaking time of the circuit breaker are inferred. Through the study on microcomputer-based protection circuit of the main transformer， the actual application in production is implemented.