With the opening of low-altitude airspace and the development of aviation technologies such as drones, the detection of low-altitude targets in areas with tight electromagnetic spectrum such as cities or suburbs has become more and more important. 5G based passive bistatic radar has a wide application prospect in this field. Compared with 4G network, the implementation details of 5G waveform scheme have undergone essential changes. Therefore, the ambiguity functions based on the signals of different external radiators are also quite different. Nevertheless, there is still lack of research on the ambiguity function of 5G signal. Based on basic structure of signal, the differences between 5G signal and 4G signal in frame structure and physical resource structure are analyzed in detail in theory. System simulation model is built, and the radar ambiguity function based on 5G signal is simulated. Finally, the cause of the ambiguity and the possible influence on the signal detection performance are analyzed, and the suppression methods of some side peaks are briefly explained. This work provides a new idea and direction for the suppression of side peaks of external radiator radar based on 5G signal.