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Exposure to higher surface air temperatures (SAT) and indoor dampness-mold can be associated with increased childhood asthma risk. However, the combined effect of heat and humidity on asthma risk, and the relationship between humid-heat and indoor dampness-mold remain unexplored. Based on a multi-city study of 37,206 Chinese preschool children, we investigated the relationships of exposure to humid-heat (SAT, heat index (HI), humidex (HMI), wet-bulb temperature (WBT)), indoor dampness-mold (dampness-mold in dwelling (DMD), and dampness-mold in clothing and bedding (DMCB)) during infancy with asthma risk. For each 1 °C increase in SAT, HI, HMI, and WBT, the odds ratio (OR) for asthma was 1.104 (95 %CI: 1.078, 1.131), 1.078 (95 %CI: 1.058, 1.099), 1.062 (95 %CI: 1.047, 1.078), and 1.085 (95 %CI: 1.063, 1.107), respectively. Exposure-response analysis showed nearly linear increasing relationships. Higher SAT, HI, HMI, and WBT were significantly associated with increased DMD and DMCB risk. Compared with those with neither DMD nor DMCB, the OR for asthma was 1.663 (95 %CI: 1.423, 1.935) among children with either DMD or DMCB, and 2.223 (95 %CI: 1.788, 2.735) among children with both DMD and DMCB. Our study highlights the importance of reducing humid-heat and indoor dampness-mold exposure during infancy to mitigate the high prevalence of childhood asthma.