Provided is a process for preparing 4-borono-L-phenylalanine, which has steps of: reacting N-protected (S)-4-halophenylalanine of formula (I), a boronating agent and an organolithium to obtain a reaction mixture, wherein the reaction mixture comprises N-protected (S)-4-boronophenylalanine of formula (II) and the R group represents a protecting group; isolating the N-protected (S)-4-boronophenylalanine from the reaction mixture; deprotecting the R group of the N-protected (S)-4-boronophenylalanine to obtain L-BPA.