The invention discloses a lost child positioning system based on big data. The lost child positioning system disclosed by the invention has the advantages that a positioning device outputs a locationsignal to a mobile phone according to the location of a child, a positioning APP stores the location signal according to the date, when the child is separated from the positioning device, the positioning device outputs an alarm signal to the mobile phone, the positioning APP outputs the location signal, the last location positioned by the positioning device and the separation time of the positioning device and the child to a cloud server, the cloud server calculates probabilities of lost direction and distance of the child, packages the probabilities into a search direction packet and outputsthe search direction packet to the positioning APP, a parent searches the child according to the probabilities of the lost direction and distance of the child. If the child removes the positioning device by himself, analysis and positioning can be performed according to the locations of the behavioral habits of the child to quickly find the lost child; and if the positioning device is compulsivelytaken off by a criminal, the police calls the surrounding monitoring videos according to the separation location of the child and the positioning device after alarm to arrest the criminal as soon aspossible so as to rescue the trafficked child.