Numerical Controlled Electrochemical Contour Evolution Machining (NC-ECCEM) is one of the most important development in Electrochemical Machining (ECM). In order to improve the machining accuracy of NC-ECCEM technology, the research works on precision NC-ECCEM technology are needed, and especially the study on its shaping law is significant for improving the machining accuracy of workpiece profile. In this paper, the shaping law of machining the planar surface by use of a kind of inner-spraying cathode with rectangle section was studied. First, the basic differential equations of shaping law in the case of cathode movement were established. Then, considering the structure of the cathode, the methods for calculating the side gap in machining the planar surface was given. Finally, the experiments of machining the planar surface were carried out. Experiments show that the calculated side gaps are bigger than the actual values, but the change trend of calculated side gaps with machining process parameters is coincident with the actual side gap change trend.