Abstract: The β-actin primer ACT1/ACT2 of sea cucumber Apostichopus japonicus was designed according to the β-actin mRNA of sea anemone Nematostella vectensis from GenBank( XM_001630533 ). RT- PCR products amplified and sequenced by this pair of primer were studied under different conditions of annealing temperature and amplified cycles. It was the first β- actin gene cloned in the sea cucumber which showed that PCR product amplified by this pair of primer can be used as endogenous control for gene semi - quantitative research.