The reservoir size sequential method is one of the commonly-used methods for petroleum resource estimation. It possesses the advantage of giving not only the total resources but also the scale and sequence of each reservoir, and the deficiency is the multiplicity of predicting results. A set of scientific methods for obtaining the key parameters of reservoir size sequential have been set up during years of work on hydrocarbon resource assessment, which including: a) using the resources that are computed by other methods (such as the genetic method) to constrain geological resources gained by reservoir size sequential method; b) using the statistical relationship between geological resources and the largest single reservoir size in mature exploration areas to constrain the largest single reservoir size in an assessed region; c) using the number of likely-existing traps(including subtle traps) to restrict the number of the predicted reservoirs. Through the use of total amount restraint, the largest single size restraint and reservoir number restraint, the assessment result overcomes problems that exist in the original method and accords better with geological conditions. The application cases of this method in Huanghekou Sag in Bohai Sea and other areas indicate that the prediction results are adapted relatively with exploration facts.