The present invention relates to a preparation method of a long-chain compound, which includes the following steps: (1) carrying out condensation reaction on H—R2 and R5N-Glu(OR4)—OR3, wherein, R3 is a carboxyl protecting group, R4 is a carboxyl activating group, and R5 is an amino protecting group; obtaining a compound of formula II; (2) removing carboxyl protecting group R3 and amino protecting group R5 of the compound shown in formula II to obtain a compound of formula III; (3) carrying out condensation reaction on the compound shown in formula III and to obtain a compound shown in formula I. The method reduces the time of deprotection, and all the reactions can be carried out in a solvent with low boiling point. The post-processing requires only simple washing and recrystallization to obtain the product with higher purity, so the method is suitable for large-scale production.