A new method based on attribute reduction, which is developed from rough set theory, is introduced to dig out main factors leading to human errors in marine accidents. The basic theory of rough set is mentioned, and a method of attribute deduction based on genetic algorithm is introduced. Experiment based on RSES, a rough set analysis tool, is implemented on a data-set which includes 30 factors which may be lead to Look-out Missing, a common human error on board. Reasonable result, an accurate set of main factors, is received after the implementation of the experiment and a conclusion that attribute reduction is a reliable and convenient way to generalize main factors lead to human errors in marine accident is made finally according to the experiment result.