A composite isolating transformer includes a main winding rack and a support rack. The main winding rack includes two winding portions coaxially formed to hold respectively a primary coil and a secondary coil, and a separating portion between the two winding portions. The support rack includes a first support half member and a second support half member coupling together to encase at least one winding portion. The first and second support half members have respectively a first cover and a second cover to cover two lateral sides of the winding portion, respectively a first insulating portion and a second insulating portion corresponding to the separating portion and extending towards the separating section to couple together, and respectively a first isolating portion and a second isolating portion extended from the first and second insulating portions towards the winding portion to cover another two lateral sides of the winding portion.