Catalase (CAT) is a main member of the plant cell anti-oxidant enzymes, which plays an important role in clearing reactive oxygen species (ROS) under the stressful conditions. In this study, the full-length sequence of CAT gene of Pyropia haitanensis was obtained by rapid amplification of cDNA ends (RACE), which was named PhCAT (GenBank accession: KM655303). Full-length of the gene is 1983 bp which could code a 542 amino acid polypeptide. The sequence similarity was 83.08% when compared with the CAT gene from P. yezoensis. Multiple sequence alignment and phylogenetic tree analysis results confirmed that PhCAT belongs to the CAT gene family. PhCAT gene quantitative analysis found the gene expression levels rose significantly when water loss equal or greater than 60%. The CAT expressions rose under high temperature stress (29℃) regardless of the treatment time. However, the responses of CAT activity showed different trends: CAT activities were significantly decreased by all levels of water loss; but only significantly enhanced under 24 h of high temperature treatment, while at the rest of the time point, enzyme activity level decreased significantly. All of these results suggested that the CAT enzyme plays an important role in responses of P. haitanensis to high temperature stress, but did not exhibit significant effects under the water loss stress.