A homemade desktop atmospheric pressure ionization time-of-flight mass spectrometer was developed,which presented an orthogonal-injection construction consisting of an ionization,an atmosphere-vacuum interface,the area of transmission and mass spectrometer whose length is 550 millimeter.Instrumental performance characteristics were obtained with the electrospray ionization.The mass analyzer provided mass resolving power of 4 200,4 900,6 000 and 7 500 at m/z 38.96,132.91,609.28 and 1 009.58,respectively.Methanol and PEG1500 ion peaks, ［CH3OH+H］+ and ［HO（CH2CH2O）39H+Na］+ were presented with m/z value of 33 and 1 758,respectively.A calibration curve was established by plotting peak area of reserpine at m/z 609 with the sample concentration in the range of 1-200 pg/μL.The detection limit（S/N=3） of the method was 1 pg/μL.As an atmospheric pressure ionization interface,the molecule ion reactor leads to ion decomposition and can be used to investigate the molecular structure.Combined with atmospheric pressure ionization,this instrument is applicable in the fields of pharmaceutical analysis,environmental analysis and food analysis,etc.