Currently, the transformer phasor diagram of on-load phasor test is generally drawn by hand in electricity industry, which has some defects, such as low efficiency of drawing, bad accuracy and non-standards drawing, etc. According to those defects, this paper presents a new method of visual interface software for phasor test, which is compiled by Visual C++ language. The implementing procedure is explained, including modal dialog box establishment, test methods selection interface, original test data input interface, phasor diagram drawing interface and three phase differential currents calculation of transformer. This method can draw phasor diagram quickly and calculate the differential current of main transformer differential protection accurately, what's more, it achieves the original test data preservation and the print function of the test report, meeting the requirements of phasor test work.