DC bias of neutral-earthing transformers is caused by earth-return current of high voltage direct current（HVDC） transmission. Using the data of power flow calculations and by building the mathematical model， computer aided analysis of DC distribution can be carried out. This approach is introduced to obtain the DC distribution and DC restrain measure of Chu-Sui ultra HVDC transmission of Guangdong power grid. Through the study， the rules of DC distribution in some random virtual power grids can be obtained. It can be concluded that more DC distribution will be resulted with larger scale power grid and more geographically concentrated distribution substations. Errors between the calculation and realistic measurement are caused by the errors in resistances of transformer windings and neutral reactor. It is observed that DC distribution of 220 kV networks is affected by the neutral-earthing 110 kV networks， but this is not the case for 500 kV systems. Utilizing neutral braking measure will lead to increment of DC distribution in local power grid， but the total amount will be reduced.