The invention discloses a compound of formula I. In the formula I R is methyl, ethyl and propyl. The invention further discloses a preparation method of the compound. The preparation method includes: allowing a compound of formula II to react with organic acid in the presence of aromatization reagent. In addition, the invention provides an application of the compound of formula I in preparation of key intermediate, namely, 2-(7-methoxyl-1-naphthyl) ethylamine of agomelatine of formula IV. The key intermediate of agomelatine is obtained by Raney nickel catalytic reduction of the compound of formula I under mild conditions (0.3-0.8MPa). By-product of reaction is low, easiness in purification is achieved, and large-scale industrial production is facilitated.