The invention provides a novel method for preparing a nano-attapulgite modified polyurethane elastic fiber. The method comprises the following steps of: dispersing nano-attapulgite in mixed solution of oligomeric dibastic alcohol and a reaction control agent a by measures such as ultrasonic and/or strong mechanical stirring, grinding and the like to form a stable and uniform colloid; then adding diisocyanate, a chain extender and a reaction control agent b respectively; and performing block copolymerization to obtain the polyurethane elastic fiber. The polyurethane elastic fiber prepared by an in-situ polymerization method has high thermal stability and mechanical property.