To realize the production of lactic acid from glycerin, a continuous hydrothermal reaction apparatus was established, and the process for continuous hydrothermal conversion of glycerin into lactic acid was studied. A yield of lactic acid approaching to that using a batch reactor could be steadily achieved under the hydrothermal conditions of 300 ℃, 10 MPa, 1.250 mol?L-1 NaOH and continuous run of 60 min. The results also showed that, with higher alkali concentration, the yields of lactic acid decreased with reaction time, which differed from the previous results using batch reactor, and that a large amount of organic acids of small weight molecule was produced abundantly, which reduced the production selectivity of lactic acid. The results were probably related to the massive specific surface area of the reactor in the continuous hydrothermal reaction apparatus, which further promoted the decomposition of the lactic acid produced. Based on the analysis of experiment results, several suggestions on the modification of the continuous hydrothermal reaction apparatus, such as the substituting material of the reactor, the protecting preheating section of the reactor and supplying alkali twice, were put forward.