In view of the deficiencies of the existing data processing methods of the Milligan oil-drop experiment, this paper analyzes the principles of the mapping method of data processing and uses the easy-to-use WPS spreadsheet application to implement the linear regression calculation of the elementary charge through its built-in LINEST function. Through the analysis of the calculation process, the iterative method is introduced to design and optimize the algorithm to improve the accuracy and precision of the calculation. Furthermore, the formula for calculating the uncertainty of the slope based on the standard deviation results returned by the LINEST function is given. Applying the method of this paper to 15 sets of 107 data from reference literature, the results of this paper were found to be more accurate with less uncertainty and relative error.