Optical down-conversion technology can simultaneously down-convert all electromagnetic signals within a wide frequency band to the low-frequency range for reception, and is a new type of fast reception technology for broadband electromagnetic environments. However, the obtained optical down-conversion signal contains multiple signals with unknown number of sources and different bandwidths. Existing signal separation methods need to know the number of source signals and cannot simultaneously separate narrowband and broadband signals. To achieve automatic separation of optical down-conversion signals, a method for optical down-conversion signal separation based on VMD adaptive mode recombination is proposed. By using spectral segmentation factors and spectral envelope detection, the VMD over decomposition modes of optical down-conversion signals are automatically recombined and signal recombination modes are extracted, achieving the separation of optical down-conversion signals. For optical down-conversion signals containing ordinary pulse signals, WCDMA signals, and linear frequency modulation pulse signals, this method can automatically separate the three types of signals, and the similarity coefficients with the original signal are all higher than 0.97. The experimental results show that the method proposed in this paper does not need to know the number of source signals when separating optical down-conversion signals, and can simultaneously separate multiple source signals with different bandwidths.