Abstract: This paper describes a method to simulate a top-hat 90° electrostatic anajyzer numerically. Computer codes for this type of analyzer are developed and applied specifically to the computation of the top-hat analyzer prototype of Sablik et al. The potential distribution in the analyzer is found from solving a Laplace equation by using finite difference method; the motion equation of charged particles leads to the paths of these particles with time being parameter; the expressions of response functions and geometric factor are correctly derived; a scaling law of the analyzer behaviors is given, and is taken into account in the entire simulation. In comparison of the present results with those of Sablik et al, the accuracy of the present simulation computation is analyzed and assessed.