The single-particle spectrum obtained from the relativistic mean field (RMF) theory is used to extract the shell correction energy with the Strutinsky method.Considering the delicate balance between the plateau condition in the Strutinsky smoothing procedure and the convergence for the total binding energy,the proper space sizes used in solving the RMF equations are investigated in detail by taking 208 pb as an example.With the proper space sizes,almost the same shell correction energies are obtained by solving the RMF equations either on basis space or in coordinate space.