Abstract: Most of the time-of-flight mass spectrometers in the world are still based on Willy-Mclaren double-field design. Choise of the parameters, however, basically depends on experiences. In this pepar, we try to solve this problem by detailed calculations and discussions, and to provide a theoretical background for the designs and improvments of time-of-flight mass spectrometers. Based on these considerations, a pulsed laser probe time-of-flight mass spectrometer has been built. The configuration and the preliminary experiment will also be brcifly introduced in this paper.