The invention discloses a device for drilling minimal borehole on one coiled tubing side with ultra-short radius, which comprises a guider, a flexible transmission shaft, a driving motor and a bit. The flexible transmission shaft is arranged inside the guider, the bit is arranged at the output end of the flexible transmission shaft, and the driving motor is arranged at the input end of the flexible transmission shaft. The flexible transmission shaft includes more than three transmission sections which are connected with each other through ball heads, and the transmission sections are in surface contact with the ball heads. The invention increases the axial area of the flexible transmission shaft by using the ball heads to connect the transmission sections of the flexible transmission shaft and designing the surface contact between the transmission sections and the ball heads, thus prolonging the service time of the flexible transmission shaft and improving the device stability.