The invention discloses a drilling tool and a method for detecting the reservoir stratum boundary and thickness while drilling. In the horizontal drilling wells of coal stratums, hydrocarbon reservoir and other reservoir stratums, an ultrahigh-frequency electromagnetic wave and microwave reflection stratum detection ranging method is applied to identify the coal stratums, hydrocarbon reservoir and other reservoir stratums and the boundaries of upper and lower overlaying stratums thereof and measure the distances between a horizontal drill and the boundaries of upper and lower overlaying stratums. When the ground trenchless construction horizontal drilling operation is carried out on the basis of the method, obstacles above and below a track of a horizontal shaft can be detected, and the distances between the horizontal shaft and the obstacles can be measured; and when the horizontal drilling operation which passes through rivers or buildings is carried out on the basis of the method, a distance between a river bed or a building foundation and the shaft below the river bed or the building foundation is detected, and the drill is kept parallel to the river bed or the building foundation or within a preset distance in a drilling process.