The invention provides an application method of a rigid drilling tool for a sidetrack horizontal well by a coiled tubing. The application method comprises the steps that on the basis of the tensile-strength safety factor of 1.4-1.5, the rigid drilling tool with the length of 600-700m is connected with the lower end of the coiled tubing, and the coiled tubing and the rigid drilling tool are loweredto a designated position in a well; under the action of 2-3.5 tons of drilling pressure, drilling is carried out for 10-20m; the return amount of rock debris at a well head is observed, if the returnamount of the rock debris at the wellhead remains unchanged, drilling is carried out continuously, if the rock debris at the wellhead is decreased, the moving drilling tool is lifted up at the wellhead, and the delivery capacity of a circulating pump is increased until all the rock debris at the bottom of the well is taken out; the pump pressure of the circulating pump is observed, if the pump pressure is increased by 1-1.5 Mpa, the moving rigid drilling tool is lifted up repeatedly so that the drilling pressure is transferred to a drill bit, and when drilling is carried out again without footage, the drilling tool is pulled out and drilling of the coiled tubing is finished. According to the application method, the problem that an effective drilling pressure at the bottom of the well gradually decreases which is caused by an existing drilling tool for the sidetrack horizontal well by the coiled tubing with the increase of the horizontal section length of the sidetrack horizontal wellis overcome, and purpose of effectively extending the horizontal section length of side tracking of a coiled tubing gas well is realized.