Aerosol laser time-of-flight mass spectrometer(ALTOFMS) generates large amounts of data during monitoring atmospheric aerosol particles, which can be used to determine the chemical composition and diameter of individual aerosol particle in real-time. A density based spatial clustering of application with noise, DBSCAN algorithm, is used to classify mixed mass spectral of aerosol of NaCl, CaCl and FeCl with an ALTOFMS. Compared with previous algorithms, DBSCAN can identify clusters with arbitrary shapes and sizes, and it produces clusters without overlap and the continua between different types. Experimental results show that DBSCAN can classify the data into three clusters. The information can provide insight into particle sources, chemical transformations between particles and gases and distribution among or within particles.