During riserless drilling for ultra-deepwater gas wells, well control challenges induced by high-pressure gas kick will be faced. A two-phase flow model with consideration of high-pressure gas invasion during riserless drilling was proposed, and the model was proved to be more accurate for predicting high-pressure gas kick during riserless drilling by reproducing field data and comparison with published models. Then, a dynamical simulation of high-pressure gas kick in ultra-deepwater riserless drilling was presented. Results showed that during ultra-deepwater riserless drilling, the bottom hole pressure will be underestimated, while pit gain will be over-estimated without considering the gas acceleration effect. With the consideration of gas acceleration effect at high-pressure well bottom, the gas influx rate increases rapidly with the kick time and tends to be stable after a period of time as negative pressure difference at well bottom increases. During riserless drilling, according to the timeliness and effectivity of kick detection methods, pit gain is prior for kick detection, following bottom hole pressure, standpipe pressure, and return rate. Moreover, if early gas kick was not detected, the rapid increase in change rate of standpipe pressure and return rate can be regarded as an indicator, showing that gas is reaching mud line. Besides, the effects of shutoff time, drilling displacement, drilling fluid density increases, geothermal gradient, and reservoir permeability on kick indicators and wellbore pressure have been discussed. The research results could provide important theoretical bases and technical guidance for well control aspects of riserless drilling.