Transport ministers from France and Spain are also expected to attend, as is Lufthansa chief Carsten Spohr.
Co-pilot Andreas Lubitz is accused of deliberately crashing the Germanwings Airbus on 24 March, killing 150 people.
A candle for each of the victims has been placed on the altar and flags will fly at half-mast across Germany.
Mourners will be invited to leave flowers on the stairways leading up to the cathedral and large screens have been erected for crowds gathered outside.
German President Joachim Gauck and Spain`s Interior Minister Jorge Fernandez Diaz are also expected at the service, which begins at 12:00 (10:00 GMT).
It will be led by the Archbishop of Cologne, Cardinal Rainer Maria Woelki and the president of the Protestant Church of Westphalia, Annette Kurschus.
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