To investigate the effects of heat stress on broiler metabolism, we assigned 144 broilers to normal control (NC), heat stress (HS) or pair-fed (PF) groups and then monitored the effects using growth performance, carcass characteristics, biochemical assays and GC-MS-based metabolomics. The up-regulation of cloacal temperature confirmed that our experiment was successful in inducing chronic heat stress. The average daily gain and average daily feed intake of the HS group were significantly lower than those of the NC group, by 28·76 and 18·42 %, respectively (P<0·001), whereas the feed:gain ratio was significantly higher, by 14·49 % (P=0·003), and heat stress also increased leg proportion (P=0·027) and intramuscular fat proportion (P<0·001) and decreased breast proportion (P=0·009). When comparing the HS and NC groups and HS and PF groups, our metabolomics approach identified seventy-eight and thirty-four metabolites, respectively, with significantly different levels (variable importance in the projection values >1 and P<0·05). The greater feed:gain ratio of the HS group was significantly positively correlated with the leg, abdominal fat, subcutaneous fat and intramuscular fat proportions and levels of some free amino acids (proline, l-cysteine, methionine and threonine) but was negatively correlated with breast proportion and levels of some NEFA (stearic acid, arachidonic acid, palmitic acid and oleic acid). These findings indicated that the heat-stressed broilers were in negative energy balance and unable to effectively mobilise fat, thereby resulting in protein decomposition, which subsequently affected growth performance and carcass characteristics.