Laser intensity noise suppression in the millihertz frequency band is essential for space-based gravitational wave detection to ensure the sensitivity of the interferometer. Optoelectronic feedback technology is one of the most effective methods of suppressing laser intensity noise. The noise of the photodetector that is the first-stage component in the feedback loop, directly couples into the feedback loop, thus significantly affecting the laser intensity noise. In this paper, starting from the requirement of suppressing laser intensity noise in the 0.1 mHz–1 Hz frequency band for space-based gravitational wave detection, the factors affecting the electronics of photodetectors at extremely low frequencies are analyzed in detail. Using the low dark current characteristic of photodiodes in photovoltaic mode, a zero-bias voltage scheme is adopted to reduce the dark noise of the photodiode. A transimpedance amplification circuit is designed using an integrated operational amplifier with zero offset voltage drift and low-temperature drift metal foil resistors, thereby optimizing the transimpedance capacitor and follower circuit to reduce 1/f noise in the circuit. Active temperature control is employed to stabilize the responsivity of photodiode, and additional measures such as using a homemade low-noise power supply and shielding interference are taken to further reduce the noise. Ultimately, an ultra-low electronic noise photodetector operating in the 0.1 mHz–1 Hz frequency band is developed. A homemade intensity noise evaluation system is used to comprehensively assess the noise both in the time domain and in the frequency domain. The constant noise characteristics of the homemade detector are estimated experimentally. The experimental results show that the electronic noise spectral density of the homemade detector reaches 2×10^–6 V/Hz^1/2 in the 0.1 mHz–1 Hz frequency band, and the electronic noise of the detector does not vary with optical power. The detector achieves a gain of 35 kV/W at 1064 nm. The noise performance of the detector is two orders of magnitude lower than the laser intensity noise requirement (1×10^–4 V/Hz^1/2) for space-based gravitational wave detection, providing a critical component and technical support for high-gain optoelectronic feedback control and laser intensity noise suppression in space-based gravitational wave detection.