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By means of the visibility measurements from July 2013 to March 2014 at Yangjiang, the difference characteristics in visibility between M6000 visibility sensor and manual observation and the causes are explored in order to improve the visibility observation quality. The results show that the difference is mostly between -20 and 30 km, and the absolute value of the difference increases with visibility; generally, the difference is normal in 90% of cases, but in bad weather, the automatic observation visibility is smaller, and the mean difference is negative and the error rate reaches the maximum when the visibility is less than 5 km. It is found that the causes of the difference, in addition to the habits and subjectivity of observers, include also the improper maintenance of the instrument and the differences between the concepts and measuring principles of the observation instruments. By strengthening the routine visibility sensor maintenance, the quality of automatic visibility observation can be improved. The automatic visibility observation can be well used to determine haze days, and in fog weather, the automatic observations can be used to make appropriate amendments to improve the consistency, so to basically meet the requirements of automatic visibility observation.