The influence of flow velocity and oxygen concentration on acetaldehyde and formaldehyde emissions from ethanol oxidation was investigated in a flow reactor under the temperature environment of engine exhaust pipe. Unregulated emissions were detected by a gas chromatography with a pulsed discharge helium ionization detector (PDHID). Experimental results showed that ethanol concentration increased with the increase of flow velocity at temperature range of 733~933K. Ethanol oxidation reaction was inhibited in anaerobic conditions, and oxygen-rich environment was conducive to ethanol oxidation. Acetaldehyde and formaldehyde emissions increased with prolonging the reaction time of ethanol oxidation when the temperature was lower than 833 K, while the opposite trend to acetaldehyde was observed when the temperature was above 833K, and the influence of flow velocity on acetaldehyde emission was not obvious when the temperature was higher than 933K. The generation and oxidation reactions of acetaldehyde were inhibited in anaerobic conditions. Below the critical temperature, the oxygen-rich environment was conducive to acetaldehyde and formaldehyde generation. Above the critical temperature, the oxygen-rich environment was conducive to acetaldehyde oxidation.