Monitronics International Inc., which operates as Brinks Home Security, is preparing to file for bankruptcy with a creditor-supported deal that would see its debt load cut by about half.

The Dallas-based home monitoring-company owned by Ascent Capital Group Inc. has a plan to reduce its debt to around $990 million through an agreement that converts $585 million of unsecured bonds to equity and pays down part of its term loan, according to a regulatory filing. The company has lined up...