The space-based gravitational wave detection can acquire the gravitational wave source information with larger characteristic mass and scale, forming a complementary detection scheme with ground-based gravitational wave detection, primordial gravitational wave detection, and pulsar gravitational wave detection. The space-based gravitational wave detection is based on a long-distance laser interference device, which mainly detects gravitational wave signals in a frequency range of 0.1 mHz–1 Hz. The noise evaluation and noise suppression of the laser light source system directly affect the detection sensitivity. In this work, based on low-noise photoelectric detection, a very low-frequency laser intensity noise test and evaluation system is constructed with high-precision digital multimeter, software control and algorithm programming of the host computer. The laser intensity noise can be converted into the fluctuation of the current signal by utilizing the photodiode, and the current signal is converted into the voltage signal and amplified by the transimpedance circuit. Thus the high-frequency interference components are filtered out by a passive low-pass filtering, and the extremely low-frequency noise components are retained. According to the definition of shot noise, it can be known that the photocurrent injected into the detector is inversely proportional to the shot noise, so at least 5 mW laser is chosen for photoelectric detection. After controlling the high-precision digital multimeter through LabVIEW software programming, the acquisition is detected. The output voltage signal by the laser is subjected to the fast Fourier transform and logarithmic frequency axis power spectral density estimation algorithm for noise evaluation in the frequency domain, forming a complete laser intensity noise evaluation and measurement system. The 0.1 mHz–1 Hz frequency band laser intensity noise evaluation is finally obtained. The experimental results show that the noise of the high-precision multimeter in a frequency band of 0.1 mHz–1 Hz is lower than 5×10^–5 V/Hz^1/2; the noise of the detector electronics ina frequency band of 0.1 mHz–1 Hz is lower than 4×10^–5 V/Hz^1/2. The electronic noise of the high-precision multimeters and the detectors meet the requirements for space gravitational wave detection. The experimental results show that the 0.1 mHz–1 Hz frequency band laser intensity noise evaluation system we built meets the needs of space-based gravitational wave detection program, and provides an important foundation for building a laser source that meets the needs of space-based gravitational wave detection.