The post-source pulse focusing(PSPF) technique was used in a linear ALTOFMS. In its focused mass range, a mass resolution of a factor of 5.7 was achieved with the optimum parameters in our experiments.The influence of various experimental parameters, such as the delay time and the amplitude of the PSPF pulse on the performance of the instrument, were discussed respectively. In addition, a new simple way, polynomial calibration, to calibrate mass number was discussed and the experimental relative error is 10^-4～10^-5 PSPF-ALTOFMS is used on-line to detect the indoor air particles.