Abstract: Single particle aerosol mass spectrometer (SPAMS) normally uses aerodynamic lens to introduce the particles from the ambient to vacuum system and focus them into a collimated beam. When exiting from the aerodynamic lens, every particle will get a velocity, which is a function of its aerodynamic diameter. Thus, the aerodynamic diameter of each particle can be calculated by measuring the corresponding velocity. However, this particle velocity is greatly influenced by the inlet pressure of the aerodynamic lens, a tiny changes of the pressure will lead to particle size calibration deviation. For the standard 740 nm PSL particles, the calculated size changes from 1680 nm to 560 nm with the pressure from 164.92 to 352.45 Pa using a fixed size calibration function, that is to say the calculated size deviates about 40-60 nm every 13.3 Pa. The inlet pressure changes from time to time during sampling period. After the long-term sampling, particles will enrich and block at the edge of the injection orifice, thus the operator need to clean the orifice from time to time to ensure the consistent pressure. Such cleaning could be very frequent when the air pollution is serious or the environmental pressure changes frequently. In order to solve this problem, this paper put forward an automatic particle size calibration method for the SPAMS. After embedding several size calibration parameters in different pressure in advance, the software could automatically fit the proper size calibration curve by the interpolation algorithm when the inlet pressure changed. This method ensured the calculated size accuracy within a certain range of pressure fluctuations. Experimental results showed that, for all the PSL particles ranging from 152 to 3 100 nm, the calculated size distribution had a peak deviation less than 2% in the range of 164.92-352.45 Pa when using this automatic interpolation algorithm. In addition, It could also be used when the elevation changes from 0 to 8 km. Actually, this method can be applied to all the similar instruments, which use the aerodynamics sampling and laser sizing technologies. In conclusion, the method is simple in operation and does not reguire any hardware to support. It can greatly increase the adaptability to external environmental conditions and improve the reliability of the measurement result of the instrument.