This paper introduces two main drawing methods of Mercator chart, and their advantages and disadvantages, and then analyzes the causes of errors and the existing limitations of the traditional simple drawing method of Mercator chart. Aiming at the problem of low accuracy when the simple drawing method is used in the case of a large latitude difference span, an improved drawing method of Mercator chart is proposed. By using the middle latitude as a parameter to calculate the average latitude magnification between two latitudes during drawing, which can effectively solve the error caused by using the average latitude calculation. Through the comparison and analysis with the method of Meridian parts, the effectiveness of this simple drawing method is verified.