A pulse gain modulation-type single-pixel three-dimensional imaging system and method relate to a three-dimensional imaging technology and are intended to meet development demands of the three-dimensional imaging technology. In the present invention, a pulse laser is used as a lighting source, a light field modulator modulates emitted light into speckle light fields, which are spatially and randomly distributed, to light a target, a unit time resolution detector collects all optical signals returned from the target, time information of the signals is encoded by utilizing a modulation gain of the detector, and the time information is accumulated by an integrator, transmitted to a computer, and stored; meanwhile, the random distribution of the speckle light fields is recorded, random speckle fields are changed for many times and recorded; and finally, a three-dimensional image of the target can be resolved. The method does not need to sample a signal at a high speed, thus greatly reducing the hardware overheads of the system, increasing the imaging speed of the system, and making the resolution ratio of the system free from limitations of a high-speed sampling bandwidth.