The invention relates to the field of genetic engineering, and in particular to a promoter and application thereof. In the invention, the PNaHsp20 promoter is cloned from nannochloropsis oceanica, and the nucleotide sequence of the PNaHsp20 promoter is shown in SEQ ID NO. 1. The promoter PNaHsp20 disclosed by the invention is capable of promoting the efficient expression of a downstream gene within a microalgae cell growth cycle, so the promoter PNaHsp20 can be applied to genetic engineering, and the promoter PNaHsp20 has a good application prospect in genetic engineering.