The formation mechanism of soil arching effect behind the retaining wall was significantly analyzed in the case of limited backfill width. It is assumed that the small principal stress arch is circular arc. Considering the limit equilibrium condition of the friction point between the retaining wall and the soil，the expression of the deflection angle of the large and small principal stress was derived. On this basis，considering the translational displacement mode of rigid retaining wall and the horizontal differential element method，the theoretical expressions of active earth pressure resultant force and intensity distribution of finite width soil behind the wall were established. Comparison with laboratory test data and previous methods indicates that the earth pressure value obtained by this method has preferably rationality. Finally，the distribution of the active earth pressure was analyzed under different ratios n of the width to height of the backfill. The results show that the active earth pressure increases with the increasing n，but approaches to a constant value as n reaches a threshold of limited width of 0.5.