Based on the tea plant transcriptome database, we cloned the fulllength cDNA sequences of the key ratelimiting enzymeNacetylserotonin methyltransferase（CsASMT）in the melatonin synthesis pathway using SMART^TM RACE from the tea plant cultivars ‘Shuchazao’ . The full length cDNA was 1 282 bp, with an ORF of 1 065 bp, encoding 354 amino acids. The predict protein molecular and theoretic isoelectric point of CsASMT are 38.98 kD and 5.37, respectively. Phylogenetic tree analysis showed that CsASMT had the closest genetic relationship with Malus zumi ASMT9. The target gene was cloned into pET32a(+) and pMalc2X vector to construct a recombinant vector, respectively, and then transformed into E.coli BL21(DE3). After they are induced by 0.5 mg/mL IPTG at 28 ℃ for 6 h, pET32a（+）/CsASMT is expressed as inclusion body protein. But, pMalc2X/CsASMT is expressed as a large amount of soluble protein with molecular weight of about 79 kD in supernatant. Quantitative realtime PCR analysis showed that the expression of CsASMT was downregulated under the treatment of Ectropis oblique feeding. Exogenous application of melatonin, ABA, MeJA and SA could significantly upregulated the CsASMT. The obtained conclusion would provide a basis for elucidating further the function of CsASMT.