As heterotrophic respiration (R(H)) has great potential to increase atmospheric CO2 concentrations, it is important to understand warming effects on R(H) for a better prediction of carbon-climate feedbacks. However, it remains unclear how R(H) responds to warming in subtropical forests. Here, we carried out trenching alone and trenching with warming treatments to test the climate warming effect on R(H) in a subtropical forest in southwestern China. During the measurement period, warming increased annual soil temperature by 2.1 °C, and increased annual mean R(H) by 22.9%. Warming effect on soil temperature (WE(T)) showed very similar pattern with warming effect on R(H) (WE(RH)), decreasing yearly. Regression analyses suggest that WE(RH) was controlled by WE(T) and also regulated by the soil water content. These results showed that the decrease of WE(RH) was not caused by acclimation to the warmer temperature, but was instead due to decrease of WE(T). We therefore suggest that global warming will accelerate soil carbon efflux to the atmosphere, regulated by the change in soil water content in subtropical forests.