||Artificial light at night (ALAN) has become increasingly recognized as a disruptor of the reproductive endocrine process and behavior of wild birds. However, there is no evidence that ALAN directly disrupt the hypothalamus-pituitary-gonadal (HPG) axis, and no information on the effects of different ALAN intensities on birds. We experimentally tested whether ALAN affects reproductive endocrine activation in the HPG axis of birds, and whether this effect is related to the intensity of ALAN, in wild tree sparrows (Passer montanus). Forty-eight adult female birds were randomly assigned to four groups. They were first exposed to a short light photoperiod (8 h light and 16 h dark per day) for 20 days, then exposed to a long light photoperiod (16 h light and 8 h dark per day) to initiate the reproductive endocrine process. During these two kinds of photoperiod treatments, the four groups of birds were exposed to 0, 85, 150, and 300 lux light in the dark phase (night) respectively. The expression of the reproductive endocrine activation related TSH-β, Dio2 and GnRH-I gene was significantly higher in birds exposed to 85 lux light at night, and significantly lower in birds exposed to 150 and 300 lux, relative to the 0 lux control. The birds exposed to 85 lux had higher peak values of plasma LH and estradiol concentration and reached the peak earlier than birds exposed to 0, 150, or 300 lux did. The lower gene expression of birds exposed to 150 and 300 lux reduced their peak LH and estradiol values, but did not delay the timing of these peaks compared to the control group. These results reveal that low intensity ALAN accelerates the activation of the reproductive endocrine process in the HPG axis, whereas high intensity ALAN retards it.