The signatories are calling on the head of the Catholic Church to "take into account the pain of the survivors", several Belgian media reported on Thursday.
According to Flandreinfo.be, a private audience with victims of sexual abuse is planned within the church.
No further details are known.
In a video published by the newspaper "Le Soir" - which also published the letter - Jean-Marc Turine (78) said that he expected a clear message from the Pope.
Turine is one of the signatories and describes himself as a "survivor". Turine called for abuse to be penalised as a crime everywhere in the world. He also questioned the mandatory celibacy of Catholic priests.
Without celibacy and with more freedom of choice for priests, some crimes might not have been committed.
Spokesman: I have the impression that those affected are being kept away
Two years ago, Turine published a book in which he describes how he was abused as a student at Saint-Michel-College in Etterbeek in the 1960s.
The Flemish priest Rik Deville, who has been the spokesman for victims of sexual abuse in the church for years, told the Belgian radio station De Ochtend that he had the impression that the victims were being kept away from the Pope's visit.
He also does not expect the Church's attitude towards those affected to change.
Pope Francis will visit Luxembourg on 26 September.
He will then be in Belgium until 29 September.