The invention relates to a low-density ablation thermal insulation type composite and a preparation method thereof. The composite adopts phenolic aerogel of the nanometer particle structure as a matrix and adopts a flexible fiber blanket or a fiber weaving body as a reinforcement body, and the composite is obtained through the steps of phenolic resin preparation, phenolic resin solution preparation, thermal treatment of the flexible fiber blanket or the fiber weaving body, steeping of the flexible fiber blanket or the fiber weaving body with the phenolic resin solution, a sol-gel reaction, aging, drying and curing of the composite and the like. Compared with the prior art, the composite is excellent in thermal protective performance, good in mechanical performance, high in designability, simple in preparation technology, low in cost and easy to process and mold, promotes later-period dimensional cutting, meets different thermal protective needs under the medium heat flow and medium and low heat flow environments, and can be applied to manned space flight and deep space detection aircrafts, outer thermal protective layers of various tactic and strategic weapons working for a short time, and inner ablation heat insulation and thermal protective layers of engines, disposable hypersonic vehicles and the like.