In the space-borne lidar, the large-aperture dual-wavelength harmonic diffractive optical system is used to achieve light-weight, and the along-track direction multi-pulse sliding window processing is adopted to improve the detection performance of the system. The 12 sub-mirrors of the optical synthetic aperture can well replace the large-aperture main mirror (Fig. 3). A single-photon array detector with a scale of 10 (along-track direction)×1000 (cross-track direction) is used. When the laser emits a narrow pulse of 5 ns, the signal-to-noise ratio per pixel is about 0 dB. The sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to about 4 dB, which can basically meet the requirements of 3D imaging. Using a laser local oscillator array detector, the signal-to-noise ratio per pixel of a single pulse is about 0 dB when the laser emits a wide pulse of 100 μs and adopts pulse compression technology, and the sliding window processing realizes the incoherent accumulation of 7 pulses, which can increase the signal-to-noise ratio to 8.4 dB.