Abstract: In this paper frozen orbit is investigated based on Brouwer's mean orbit elements .It has been found that the orbit elements of frozen orbit describe an exact circular orbit.Furthermore,a new explanation to the characteristics of frozen orbit is presented.In addition,the variations of orbit eccentricity corresponding to the changes of orbit inclination is analyzed under the earth gravitational field including only J 2 and J 3.It can be seen that the abnormal behavior near critical inclination is still kept.