This article introduces a down-hole inductively coupled contactless power transfer device for oil well drilling.Its primary cot and circuits are fixed on the rotary parts of the down-hole tool while its secondary coil and circuits are fixed on the non-rotating parts.There are small gaps between the primary and secondary parts while are filled with fluid flow.This device transfers the input DC power of the primary side to the secondary side to provide electrical power to the other secondary devices.Experiments have proved that this device can guarantee steady power transfer in 150℃ fluid and its efficiency can be as high as 65% with a 300W input.Oil well drilling tools equipped with this device have accomplished 14 well drilling operations with the footage more than 10000 meters.The stability and reliability of this device have been proved, and it has brilliant application prospects.