Abstract: The measurement is based on Brophy's method, but the circuit of timegenerator is modified, and the method of curve fitting is used for the massnumter determination of mass spectra. In the paper, three problems are des-cribed: (1) the design of circuit of time trace generator; (2) the analysis ofsystem error; (3) the computer programming. This method can be used todetermine accurate mass number with low resolution. The error is less than0.008 a. m. u. It is recommend especially to those who possess a mass spectro-meter with resolution of 1000 or higher as a means of obtaining element compo-sitions.