In this paper, we present the method for optical simulation that we have successfully applied to new designs of fire detectors, with a focus on optical scattering smoke detectors. Our simulation is based on a commercially available software tool for non-sequential ray-tracing and on measurements of the differential scattering cross-section of test fire aerosols. With optical simulation, we succeeded in predicting the main characteristics of an optical scattering smoke detector, the zero-smoke signal and the response signal to test fires with sufficient precision.