After immunizing Bactrian camel with CRP as immunogen,the peripheral blood lymphocytes were isolated,the total RNA was extracted,and then the cDNA was obtained by reverse transcription. The target gene fragment was obtained by nested PCR and cloned on the arm of T7 vector of phage to construct the original phage library. After four rounds of biological panning,the phage library after panning was identified by Phage-ELISA. The recombinant plasmids of pET-28a and positive clones were constructed by BamH Ⅰ and HindⅢ double enzyme digestion and transformed into BL21 strain for low-temperature induction expression. The single domain antibody was purified by His-Ni affinity chromatography,and then the reactivity between the single domain antibody and CRP was identified by indirect ELISA. The results showed that the phage library with a capacity of 1.08×108 cfu was successfully constructed,and the gene insertion rate was 96.43% (27/28). Four positive clones were successfully constructed and expressed in BL21 cells. The molecular weight was about 22 ku. By ELISA,the four antibodies showed high affinity and specificity. The titer of V2 and V3 was 1∶3 200,which proved that the antibody could be applied to the development of practical detection methods.