While attending Seoul Physical Education High School in 1990, Sim was first selected for the South Korean national wrestling team.

Sim first gained attention at the 1993 World Wrestling Championships where he won the bronze medal in the 48 kg category.

While trying to earn his spot to compete for the 1992 Summer Olympics, in Barcelona, Sim was ranked first in the Men's Greco-Roman 48 kg at the South Korean national trials, beating 1991 World Champion Goun Duk-Yong.

Amid controversy, however, Goun was selected by Korea Wrestling Federation over Sim in the 48 kg category at the Barcelona Games, despite being ranked lower in the trials.

At the Atlanta Games in 1996, Sim won his first Olympic title by defeating Aleksandr Pavlov of Belarus in the final.