Six are charged with attempted murder, accused of setting fire to newspaper with which the man had covered himself.
The seventh is charged with failing to assist the Polish man, who was only saved by the actions of passers-by after his attackers fled.
The attack, which was captured on CCTV, shocked the country.
Six of the accused come from Syria, a seventh from Libya. They range between 16 and 21 in age.
They arrived in Germany between 2014 and 2016, some as unaccompanied minors, say German media reports.
The case has renewed debate about the care of unaccompanied minors, as well as adding to an ongoing argument about whether CCTV cameras help combat crime.
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The attack took place in the Schoenleinstrasse underground station in the southern Berlin district of Neukoelln at about 02:00 (01:00 GMT) on Christmas Day.
The alleged ringleader, a 21-year-old Syrian, is accused of lighting a handkerchief and placing it near the man's head, and then running away as the flames spread to the man's backpack and a plastic bag.
A train driver used a fire extinguisher to control the flames as passers-by helped to put them out.
Six of the suspects surrendered to police immediately after CCTV pictures of the incident in Berlin were released.
The alleged ringleader was arrested near his home.
He now says he was drunk and cannot remember what happened.
But he will be charged as an adult and will face at least three years behind bars if found guilty.
They have all been in detention since 27 December.
Seven young migrants have gone on trial in Germany, over a Christmas day attack on a homeless man which could have killed him.