The rate of spontaneous emission of strontium intercombination transition between singlet state and triplet state (5s2)1S0-(5s5p)3P1 is less than the rate of electric dipole transition. Compared with the dipole transition, the fluorescent signal of intercombination transition is very weak. A bias detector is reported which is used to detect the fluorescent signal of this intercombination transition. An ultralow input bias current operational amplifier is applied in this detector to pre-amplify the weak signal, which makes the signal have high signal-to-noise ratio (SNR) and high gain. The detector′s bias can be adjusted. Its gain is on the order of 106 V/W and -3 dB bandwidth is about 1 MHz. According to our experiment, the weak sympathetic fluorescent signal of (5s2)1S0-(5s5p)3P1 intercombination transition is detected by the detector. Consequently, we obtain the fluorescent signal with high SNR and zero bias. Also we detect the high-SNR error signal of the fluorescent signal. Besides, saturated fluorescence spectrum and its error signal are also acquired by the detector. These signals can satisfy the requirement of 689 nm laser frequency stabilization and then be used for strontium optical clock.