A data fusion based safety surveillances system has a first through Nth virtual monitoring systems and a data fusion and decision device. The data fusion and decision device defines an ith detection model of the ith virtual monitoring system, and estimates an ith detection loss probability of the ith virtual monitoring system according to detection loss sample numbers of batches of ith monitored data of the ith virtual monitoring system under conditions corresponding to the locations, batches of context data and existence of intruder, the ith detection model, the batches of ith monitored data corresponding to locations of the ith virtual monitoring system and the batches of context data. The data fusion and decision device determines a fusion parameter set according to the first through Nth detection loss probabilities, and performs data fusion on a first through Nth detection results to generate a decision result.