Humidity is a significant factor contributing to heat stress but it is not fully considered in studies quantifying heat stress or in heat risk assessment. It is directly related to human body heat exchange and the co-occurrence of consecutive hot and humid days during a heatwave can strongly affect human health. In this study, wet-bulb globe temperature (WBGT) considering both temperature and humidity effects was utilized as a heat index to define heatwaves. Using daily mean air temperature and relative humidity data from 824 meteorological stations for the reference period (1986-2005) and the corresponding CMIP5 multi-model simulations, spatial distribution characteristics and change of heatwaves that would occur in China's mainland were analyzed for the future period (2076-2095) under different greenhouse gas emission scenarios (RCP2.6, RCP4.5, and RCP8.5). Our analysis suggests that the average number of heatwave days in a year would be 3.4 and 6.6 times of that for the reference period under the most aggressive mitigation scenario (RCP2.6) and high emission scenario (RCP8.5), respectively. Average heatwave amplitude (as defined by the peak daily WBGT in the hottest heatwave in a year) would increase 4.9 ℃ under RCP8.5 as opposed to about 1.6 ℃ under RCP2.6. In the future period, the highest annual heatwave amplitude of eastern and southern China would reach 40 ℃ under the RCP8.5 scenario, which is higher than the optimum body core temperature (near 37 ℃). Although the Tibet Plateau has low heat amplitude, increase in the annual total heatwave days is rather significant in the future period. Heatwaves in the future would be most serious over southern China, the middle and lower reaches of the Yangtze River and parts of southwestern China considering both temperature and humidity effects on human thermal comfort. It suggests that without taking surface air humidity into consideration, there could likely be an underestimation of intensity and influences of heatwaves over areas with high humidity (such as southern and eastern China).