Abstract: Ambient mass spectrometry enables the direct analysis of trace samples in real time at ambient condition without sample pretreatment. In recent years, various ambient ionization methods have been developed based on different ionization mechanisms. Ambient photoionization mass spectrometry was developed on atmospheric pressure photoionization, which has the advantages of no ionization polarity discrimination, soft ionization, and reduced ion suppression from matrix effect. Normally dopant needs to be added in ambient photoionization mass spectrometry to enhance the ionization efficiency. Target analytes can be directly ionized by photoionization, or ionized by ion-molecule reactions induced by photoionization. The gas, liquid and solid chemicals in complex matrix can be directly analyzed by ambient photoionization mass spectrometry, which has been widely applied in the analysis about environmental pollution, national security, clinical medicine and life sciences. This work explains the photoionization mechanism and discusses the principles and applications of some representative ambient photoionization mass spectrometry methods.