Abstract: A KDY-130 borehole multi-extensometer developed recently was briefly introduced; and its application on measuring the deformation of surrounding rock of diversion tunnel in Lubuge Hydraulic Power Station is described. It is proved that the ex-tensometer has advantages in its reliability, high sensitivity, anti-vibration and remote measuring; the apparatus can be used for determining the changes of the deformation with the depths and rheology properties of surrounding rocks as well as the space effects of excavation working faces of tunnels.By analyzing the data measured, the deformation bahavior of surrounding rock has been found out, after comparing them with those in literatures and with some theo retical results the reliability of the testing results has been examined.