The finite element method has been applied to calculate the 50Hz electric fields nearby the transmission lines. The electric field strengths nearby multi-circuit transmission lines on the same tower under different arrangement of phase sequence, nearby the cross-arm are calculated. According to the foreign and domestic security standards of 50Hz electric field, the impacts of overhead transmission lines on environment are evaluated and the adoptable measures to reduce electric field strength are proposed. Meanwhile the distributing pattern of the electric field nearby the cross-arm is analyzed and proposals for further protection measures during maintenance operation are provided.