Tsvetan Tsvetanov, 32, was brought to the hearing on Thursday in handcuffs -- both on his hands and feet -- and under heavy police escort.
He was arrested in his home in a Roma neighbourhood of Lom in northwest Bulgaria on Monday on a European arrest warrant issued by Austrian prosecutors.
Appearing in court in the Bulgarian town of Montana, he kept silent when asked by journalists if he was the driver of the truck but answered "No" when asked if he knew that he was carrying people inside.
According to the charge sheet he allegedly drove the truck, found on an Austrian motorway near the Hungarian border last Thursday, "at least some of the distance from Hungary to Austria."
The court was due to examine an Austrian extradition request on Monday.
The charges include "participation in an organised crime group, contraband trafficking and premeditated manslaughter of 71 people," Montana regional prosecutor Nina Borisova told the court.
Another five men, four Bulgarians and an Afghan, have so far been detained in Hungary.
Austrian police are due to release autopsy results on Friday of the 71 men, women and children, thought to be Syrians and to have suffocated up to two days before the truck was discovered.