The invention discloses an online pH adjusting device and an online pH adjusting process of catalytic hydrogenation reaction. The online pH adjusting device comprises a material cooling system, an online pH detection unit, an alkaline liquid supplementing unit and a control unit, wherein the material cooling system is communicated with a reaction kettle, and a material circulating loop is formed between the material cooling system and the reaction kettle; the online pH detection unit is used for detecting a pH value of a material cooled by the material cooling system; the alkaline liquid supplementing unit is used for conveying alkaline liquid into the reaction kettle; the control unit is used for receiving a signal of the online pH detection unit and controlling the alkaline liquid supplementing unit to be started and stopped according to the signal. The material cooling system and the reaction kettle form the material circulating loop, the pH value of the cooled material on the material circulating loop and adjusted according to the measurement result, and the variation of the pH value in the reaction process can be monitored in real time, so that the pH value in the entire catalytic hydrogenation reaction process is stable, the high efficiency of the catalytic hydrogenation reaction can be guaranteed, automatic control for the entire process is adopted, and the operation difficulty of workers can be alleviated.