Monitoring the actual state of transformer is imperative for measuring the errors of transformer with its actual burden load. The simplified three phases testing method is the practical and effective in monitoring the transformer state. And a programmable three-phase power supply is its core. The three-phase programmable power supply using DDS synthesizer can change the phase angle through changing triggering time of frequency update and adjusts amplitude through multiplier. It can generate three-phase signal voltage whose amplitude and phase angle can be adjusted by program. So it is suitable for the instrument for measuring the errors of transformer with its actual burden load. By using the instrument an environment of actual burden load is simulated completely. So the measuring of transformers with actual burden load is made precisely. The measuring principle and hardware and software design of the instrument are discussed in the paper.