Abstract: The existing production line still relies on empirical process parameters as a guide for the cold assembly process of 20CrMnMo alloy steel workpieces after quenching, leading to a lack of systematic design methods for the cold assembly process equipment. It is urgently necessary to reveal the time-varying laws of workpiece temperature and deformation during assembly. This article conducts experimental studies on the low-temperature thermal properties of 20CrMnMo material after quenching. Based on this, numerical investigations were carried out on the temperature and deformation variations over time for several typical workpieces in the cold assembly process. The results showed that under the same operating conditions, the rates of workpiece temperature change and deformation increased with the size of the workpieces. The time-varying laws of workpiece temperature and deformation were confirmed through experiments, and the simulation results were found to be in good agreement with the experimental results. Based on the machining errors of the workpieces and the precision requirements of assembly, accurate prediction of assembly time for the cold assembly process can be achieved. The research findings will provide a theoretical basis for the study of material thermal properties and the precise setting of key parameters for the cold assembly process.