According to the Virtual Nuclear Experiment lab reports of three consecutive grades undergraduatestudents of the Modern Physics Experiment of the Institute of Physies and Astronomy at Sun Yat-senUniversity , it was found that when students used mathematical formulas to caleulate the thickness of the Mylarfilm , there was a large error with the theoretical value.The average relative errors of each grade are 445.02%,335.45%6 and 519.92%6 respectively.In order to find the reason to cause such huge errors, a real nuclearexperiment was conducted and mathematical calculation methods were still used to calculate the Mylar filmthickness.lt was found that there was still a large error with the theoretical value ,and the relative eror valuewas 207. 72%6. When Srim software is used to calculate the Mylar film thickness in the virtual nuclearexperiment and the real nuclear experiment , the relative errors are very small ,7.47%6 and 20.1%6 respectively.ltis thus determined that when using mathematical combination formulas to caleulate the polyatomic compound mylar film , the inaccuracy is high and the error is large.