1992 Olympic trials

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 Barcelona Games Goun was eventually eliminated in Round 1.

1996 Olympics

At the Atlanta Games in 1996, Sim won his first Olympic title by defeating Aleksandr Pavlov of Belarus in the final. He scored two points with a chest-high roll-through with 42 seconds left in regulation time and then added two more points with another roll in overtime to score a 4–0 victory.

In November 1996 Sim became the last world light flyweight (48 kg) champion at the 1996 World Cup where the FILA's final international 48 kg class competitions were held, dominating all the opponents by technical fall including two-time World Champion Wilber Sánchez of Cuba.

2000 Olympics

Sim moved up in weight from 48 kg to 54 kg in 1997 when the new weight classes were established by FILA. In the semifinals of the Sydney Games in 2000, he defeated Kang Yong-Gyun of North Korea 10–0. At the Opening Ceremony, the North and South Koreans had marched together. In this spirit, before the medal matches, Sim gave advice to Kang about the man he would be facing in the bronze-medal match-Andriy Kalashnikov of Ukraine, while Kang gave Sim a scouting report on his opponent in the gold-medal match-Lázaro Rivas of Cuba. Both Koreans won, with Sim scoring early and often against Rivas and prevailing 8–0.