Transformer calibrators are a kind of equipment used to test the transformer error in the power system. With the development of technology, the transformer calibrator is also constantly innovating, from manual, and automatic to intelligent development, which has become a research hot spot. Two types of calibrators according to the usage method, namely the difference measurement method and the direct comparison method were introduced and the advantages and disadvantages of each method and related products were analyzed. Currently, digital calibrators had became a research hotspot because of their high degree of automation and multiple functions. The digital calibrator could be used for both conventional and non-conventional transformers, with a wide range of applications. Three key technologies that affect the accuracy of the digital calibrator are focused on by the paper. And a new technology based on small-signal on-site calibration were introduced, which had significant advantages over the current commonly used on-site calibration methods, with strong compatibility and high security, making the transformer calibration device smaller and more portable, and suitable for a wide range of occasions. Finally, it was pointed out that the transformer calibrator should continuously develop towards digitalization, intelligence, portability, accuracy improvement, and comprehensive performance improvement.