In order to explore the mode of orthogonal Porro prism, the output mode of an orthogonal Porro prism resonator is investigated on the basis of the Fresnel-Kirchhoff diffraction integral formula and is simulated by using the fast Fourier-transform algorithm. By analyzing the manufacturing errors in different apex widths and right angles of these prisms, the conditions for maintaining the single-transverse-mode operation of the orthogonal Porro prism cavity are determined. The simulation results show that the output spot integrity can be guaranteed when the machining error of Porro prism is below 18 μm and the angle error is below 2″.