Different dual-metal ions were selected as catalysts to prepare lactic acid (LA) by hydrothermal conversion of glucose under alkaline conditions, and Sn$^{2+}$ and Cu$^{2+}$ were found to have a certain synergistic effect. The effects of reaction temperature, time, alkali addition amount, and metal ion ratio on lactic acid yield were studied to optimise the experimental conditions. The optimum process conditions were as follows: 0.025 g glucose, 2 mmol sodium hydroxide, $n$(Sn$^{2+})$:$n$(Cu$^{2+})$=0.25:0.75 (1 mmol total), 20 mL water, reaction temperature 200 ${^\circ}$C, and reaction time 3 h. Under optimal conditions, the yields of lactic acid, formic acid (FA), and acetic acid (AA) were 64.1%, 20.3% and 11%, respectively. Several model compounds were selected as intermediate products for the hydrothermal transformation experiments. According to the experimental results, the conversion mechanism of the hydrothermal conversion of glucose was investigated. Important intermediates detected in the conversion process were fructose, 1,3-dihydroxyacetone and pyruvaldehyde. There was a synergistic effect between the two metal ions, as Cu$^{2+}$ inhibited the decomposition of lactic acid, and Sn$^{2+}$ increased the conversion efficiency of pyruvaldehyde to lactic acid, finally increasing the yield of lactic acid.