High temperature in summer is a key barrier for Bradysia odoriphaga (Diptera: Sciaridae), a serious pest of vegetables in China. Because mild heat hardening can enhance the thermotolerance of many insects, we determined if heat hardening was beneficial for B. odoriphaga adaptation to high summer temperatures. We studied the biological performance, antioxidant responses and hsp70 and hsp90 expression of B. odoriphaga 4^th instar larvae and adults after pre-exposure to mild heat hardening temperatures (30-36°C). The tolerance of larvae to severe heat stress was enhanced after mild heat hardening (32-36°C). However, life parameters were significantly inhibited, development was delayed, longevity was shortened and fecundity was decreased. Mild heat hardening (30-34°C) caused serious adverse effects on life parameters and less enhancement of heat resistance for 4^th larvae. Mild heat hardening at 36°C weakened the heat tolerance of adults. The activity of four antioxidant enzymes in B. odoriphaga changed dramatically after heat hardening at 32-36°C. CAT and GST maintained successive higher activities, and POD also had higher activities in larvae after 48 h recovery at 25°C, pupae, and adults. Simultaneously, the expression levels of hsp70 and hsp90 increased significantly upon thermal hardening. Higher expression levels were also detected in larvae after 48 h recovery at 25°C, pupae, and adults. Mild heat hardening played an important role in heat tolerance plasticity of B. odoriphaga larvae. The antioxidant system and hsp70 and hsp90 genes were relevant to this adaptation process.