In order to analyze the connection between space charge and electric field under high voltage direct current transmission line, Sarma method is used for numerical calculation to analyze the connection between space charge density and electric field under unipolar direct current lines. On the basis of space charge density measuring device, the data is tested under the actual line. The influence of distribution law is studied for some circuit parameters of space charge density and electric field. The results show that space charge density and electric field has a similar change rule. Increasing the height can reduce the space charge density and electric field obviously, but line spacing and split number have little influence on them. Air quality is an important factor to affect the size of the space charge density and total electric field.