Objective To simulate the chemical microenvironment of traumatic brain injury (TBI) under mild hypothermia, and investigate the effect of such microenvironment on umbilical cord mesenchymal stem cells (UCMSCs) in vitro.Methods Eighteen SD rats were allocated to shamoperated group, TBI group and mild hypothermia group according to the random number table, with 6 rats per group.Rat models of TBI were made by electric cortical contusion impactor.After systemic mild hypothermia (33℃) for 4 h, brain tissue homogenate extracts were harvested.Polyacrylamide gels mimicking the elastic modulus of brain were manufactured.Human UCMSCs were isolated and cultured on the gels, added with brain tissue extracts from each group.After 24 h, the apoptosis level of UCMSCs was checked, and the medium was changed with normal one.Cell growth and morphological changes in each group were given dynamic observation.Seven days later, cell immunofluorescence was implemented, with the differentiation level of each group estimated.Results Apoptotic rate in TBI group was 73.47％,significantly higher than 10.42％ in sham-operated group (P ＜0.01).While the apoptotic rate was 28.57％ in mild hypothermia group, indicating mild hypothermia significantly reversed the apoptosis of cells in TBI group (P ＜ 0.01).Cell immunofluorescence demonstrated rate of neuronal differentiation of UCMSCs in sham-operated group, TBI group and mild hypothermia group was 16.48％, 2.59％ and 11.83％ respectively.Mild hypothermia resulted in significantly improved neuronal differentiation of UCMSCs after TBI (P ＜ 0.05).Conclusions More apoptosis and lower neuronal differentiation ability are observed in UCMSCs in the chemical microenvironment after TBI.However, mild hypothermia significantly reverses the elevation of apoptosis and restores the neuronal differentiation capacity of UCMSCs after TBI.