The analysis model of a semi-submergible drilling platform with Dynamic Positioning (DP) system is established by MOSES software, and the coupled dynamic characteristics of the platform under the action of internal waves during drilling operation are simulated in time domain. The station keeping capacity of the platform is evaluated under internal wave conditions with different velocities, and the effects of some key factors of DP system are studied by sensitivity studies. The results show that: under the same environmental load intensity, the maximum deviation occurs in the port/starboard direction of the platform; the platform can satisfy the stationing requirement for drilling operation when the velocity of internal wave is lower than 1.0 m/s, when the velocity of internal wave reaches 2.0 m/s, large horizontal offset and trim angle emerge and the drilling riser has to be released; the station keeping capacity of the platform will be enhanced by increasing the thruster force or sensitivity factors of thruster force but the offset can not be suppressed efficiently. Therefore, it is recommended to evacuate the platform or adjust yaw angle when the platform encounters internal wave condition with high velocity.