Self written DP analysis program is used. Taken a drilling ship as an example, the internal solitary wave is included in the environmental load for DP capability calculation and the result is compared with that of the traditional DP capability. According to the calculation results, DP capability curve is drawn, and the working window of the drilling ship is obtained. It is found that the internal solitary wave is of a great influence on the DP capability of the drilling ship, and the most obvious impact is in the wave direction of 30°,45° and 135°,150°. DP capability of the drilling ship is evaluated in combination with the failure condition of the propeller, and the suggestion of the offshore operation of the drilling ship encountering the internal solitary wave is proposed.