Abstract: The bottomhole pressure control technology is one of the key technologies in underbalanced drilling.Research on three key technologies is carried out in this paper to have a precise knowledge of the bottomhole pressure and improve the precision of controlling the negative pressure of bottomholes.Firstly,a new kind of measurement instruments with antiseismic function and resistant to the high temperature of 180℃ is developed on the basis of using the measurement while drilling(MWD) method.Secondly,a new model for calculating the bottomhole pressure of multiphase flow in annulus in flow-drilling under balance pressure drilling is suggested based on its original model and taking the acceleration drop of solid and multiple phases flow into account.The computational formula on pressure increment in bottomholes affected by drilling speed is deduced as well.Finally,the calculation software is developed for monitoring and analyzing drilling to testify the precision of the new model.The calculation result is less than 3% as compared to the experimental data.The research reveals that these three key technologies can improve the accuracy to calculate and control bottomhole pressure,providing theoretical foundation and practical data for calculating and precisely controlling underbalanced design.