A stray radiation multi-scale simulation method of a complex spatial optical detection system relates to the field of stray radiation of optical detection systems. The Monte Carlo method of light radiation propagation is used as the basic calculation principle. A modeling is performed under a multi-scale spatial geometric structure and energy levels, so as to perform a simulation calculation to spectral characteristics and energy level information characteristic identifiers. In the calculation process, inter-scale coupling relationships are introduced as bonding points of geometrical characteristics and optical characteristics under different scales. These coupling relationships include regional virtual interface light radiation characteristics, interface energy level division numbers and sampling probability models, and bidirectional reflectance distribution functions of micro-structural stray equivalent surfaces. Then a multilayer domain decomposition technology, a multilevel light division technology, and a reverse/bidirectional Monte Carlo method light tracing technology calculation method are comprehensively utilized for achieving the effective simulation of the multi-scale stray radiation transmission of the spatial optical system. The stray radiation multi-scale simulation method of the complex spatial optical detection system can be applied to the simulation of the multi-scale stray radiation of the optical detection systems.