The war-torn nation was awarded Test status in June, along with Ireland.
The appointment was announced on Twitter by the Afghanistan Cricket Board.
The former opening batsman and medium-fast bowler, who retired in 2002, had previous coaching spells with Zimbabwe, Ireland and the West Indies.
Simmons will now join the Afghanistan squad in Dubai as it prepares for a limited-overs series against Zimbabwe in Sharjah in February.
Afghanistan will play their first Test match against India, where they play the majority of their home games, although a date has yet to be announced.
Khan has produced eye-catching performances in the world's top T20 leagues, most recently with a man-of-the-match display on his Big Bash debut for the Adelaide Strikers.
"I play cricket to bring victories for my country and to make Afghanistan proud on the world stage," he told AFP in an interview in October.