A high voltage transformer for a backlight power source includes a windings base, a core and windings, the windings base having isolating walls disposed exterior thereto through which a primary side region and a secondary side region are formed, the secondary side region being optionally formed with several windings troughs by using of the isolating walls and the windings being wound on the windings troughs, the windings being wound upward layer by layer on a bottom of the windings trough when the windings are wound across the isolating walls in prevention of the windings of different voltages flown therein crossing and contacting with each other and fixed onto a windings fixation post after the windings troughs are full, and the core being received within a hollow structure of the windings base. As such, distance generated from stacking of the windings is served to increase the bearing voltage and reduce length of the transformer.