Extensometer is a device to measure the strain. The paper is to introduce the constituent parts,theory, advantages and disadvantages and verification of extensometer. According to the verification regulation the main regulator of extensometer is calibrator of extensometers.But limited to the extensometer type and laboratory condition,we can use some other equipments to calibrate extensometers.The paper is to introduce the ways which can calibrate the extensometers except for using the calibrator of extensometers.We can use the grating dial indicator gauges or block and micrometers instead of extensometer calibrator. And it also mentions the extensometers estimating, parameter setting and repairing. At last, the paper proposes non-contact auto extensometer is the development direction in the future.