The invention relates to a fixing device used for fixing the back skin of an experimental animal. The fixing device used for fixing the back skin of the experimental animal comprises a base and fixing tables, wherein each fixing table comprises a connecting part and a fixing and clamping part connected with the connecting part, the fixing tables are connected with the base in a detachable mode through the connecting parts, the fixing and clamping parts of the two fixing tables are matched to form a fixing through hole allowing the experimental animal to pass through, and the end faces, away from the connecting parts, of the two fixing and clamping parts are matched to clamp the back skin of the experimental animal so that the back skin which is pulled up of the experimental animal can be spread on the outer walls of the fixing and clamping parts. The fixing device used for fixing the back skin of the experimental animal has the advantages that the back skin of the experimental animal can be fixed, negative influence of respiratory movement of the experimental animal on imaging is effectively avoided, damage to the experimental animal is small due to the fact the non-invasive fixing method is adopted, and long-time and stable observation and study of a subcutaneous transplantation tumor model on the back of the experimental animal by means of a photoacoustic imaging system is available.