We develop a method to compute the segment size in the detrended fluctuation analysis (DFA), which is based on the basic concept of the information theory, and verify the method effectiveness by numerical experiment. This method is freed from the problem of subjectivity in the former process to choose the segment size which usually leads to false result. We Change the length of sequence with dynamics being the same, the results remain stable. The results indicate that when the length of sequence is too short, even the optimal selection of segment size is not enough for the portrait of the overall dynamic system, thus the DFA cannot be used in this circumstance. The method we developed in this paper can enhance the reliability of DFA results by judging whether the sequences analyzed meet the requirements of DFA. We also obtain the DFA index from 1961 to 2000 of China through DFA method and analyze its spatial characteristics of distribution.