A full continuous-flow preparation method of L-carnitine, including: mixing chlorine gas and a diketene solution via a first micromixer followed by transportation to a first microchannel reactor for continuous chlorination and esterification reaction to obtain 4-chloroacetoacetate; feeding the 4-chloroacetoacetate and a reductase to a second micromixer and a second microchannel reactor in sequence for continuous catalytic reaction to obtain (R)-4-chloro-3-hydroxybutyrate; simultaneously transporting the (R)-4-chloro-3-hydroxybutyrate and a trimethylamine solution to a third micromixer and a third microchannel reactor for continuous substitution and hydrolysis reaction; and subjecting the reaction mixture to desalination and concentration to obtain the L-carnitine.