We propose an improved method to calculate the energy deposited in resist for electron beam lithography simulation. The concept of the method is that an exposure intensity distribution (EID) function is partitioned into rings having a constant contribution. The equi-contribution partitioning (ECOP) method decreases the partitioning ring number for the expression of EID and the calculation time dramatically. Using the experimental result of the EID at an acceleration voltage of 100 kV for various patterns, the ECOP method was evaluated for calculation accuracy and calculation time. The time required in this method with a ring number of 50 is ten times shorter than that in the conventional method with the number of rings over 600 while the accuracy obtained by this method is better than that by the conventional method.