This paper mainly discusses the validity and the reliability of the stray light calculation code using the Monte Carlo method in an optical system. As a new method, the symmetrical test of radiation in two-dimensional media (STRTDM) is presented to verify the characteristics of pseudorandom numbers. The method can provide great flexibility and simplicity under various calculation conditions through iteratively comparing the performance of the pseudorandom number generators (PNGs) until a sufficiently good PNG is obtained. After a series of tests have been performed—including tests for the characteristics of a PNG, for the reliability of probability models, and for the summation reciprocity relationships of the radiative exchange factor—the results show the reliability of the stray light calculation code in this paper. Moreover, the Monte Carlo computer code is verified by comparing its predictions with previously published results for optical geometries similar to ours.