Codon bias of chloroplast genome in Castanopsis carlesii was analyzed by CodonW 1.4.2 and online software CUSP,and 50 chloroplast genome genes were screened and divided into 7 classes.The results showed that the average GC content of the third codon in the chloroplast genome of C.carlesii was 29.03%,and 31 codon RSCU values were greater than 1,of which 29 codon ended in A or U,indicating that this species preferred to end in A or U.The ENC range was 38.42 to 56.53,the mean value was 48.05,and the CAI mean value was 0.168,indicating a low codon usage bias.Neutral plot analysis,ENC-plot analysis and PR2-plot bias analysis all showed that selection pressure was the main factor affecting codon preference.Twelve optimal codons were select-ed,nine of which ended in U or A.The phylogenetic tree constructed by CDS sequence and the cluster tree con-structed by RSCU value have some similarities,indicating that codon preference is related to the evolutionary rela-tionship of species.