The invention relates to a double-top drill bit with a drilling display function and applicable to radial borehole casing windowing. The double-top drill bit comprises a drill bit body and a drilling display ring; the drilling display ring is made of easy-grinding materials and sleeves the drill bit body from the root and with the diameter larger than the external diameter of each drill bit cutting edge. When the double-top drill bit is used for casing windowing, firstly, the casing wall is drilled through the cutting edges at the top of the drill bit, and the two cutting edges are operated in a matched manner till a hole meeting design requirements is drilled out. When windowing of the drill bit meets the diameter and certain depth which are required in design, the drilling display ring contacts with the casing wall, drilling is performed continuously, and the part, larger than the outer diameter of each drill bit cutting edge, of the drilling display ring can be ground off, so that windowing can be judged to be successful, and accuracy and reliability are achieved.